Centralize configuration defaults

[st0012]

Currently, we use CONFIG[:key] || default to apply default values at the places we use the config. This has 2 problems:

1. The config descriptions' default and the actual default needs to be synced manually. And as the time goes by, some of them are likely to be out-of-sync gradually.
2. Config defaults are scattered around the codebase and you need to know where a config is used to see its default. With code searching this isn't a big issue but it's still nicer to see all of them in one place.

So in this PR, I moved all configs' default to the config.rb, just right after the config type info. And now the default is synced between README <-> config command <-> actual logic.

Changes on the config command output

Before

(rdbg) config    # command
log_level = (default)          # UI: Log level same as Logger (default: WARN)
show_src_lines = (default)     # UI: Show n lines source code on breakpoint (default: 10 lines)
show_frames = (default)        # UI: Show n frames on breakpoint (default: 2 frames)
use_short_path = (default)     # UI: Show shorten PATH (like $(Gem)/foo.rb)
no_color = (default)           # UI: Do not use colorize (default: false)
no_sigint_hook = (default)     # UI: Do not suspend on SIGINT (default: false)
no_reline = (default)          # UI: Do not use Reline library (default: false)
skip_path = (default)          # CONTROL: Skip showing/entering frames for given paths (default: [])
skip_nosrc = (default)         # CONTROL: Skip on no source code lines (default: false)
keep_alloc_site = (default)    # CONTROL: Keep allocation site and p, pp shows it (default: false)
postmortem = (default)         # CONTROL: Enable postmortem debug (default: false)
fork_mode = (default)          # CONTROL: Control which process activates a debugger after fork (both/parent/child) (default: both)
sigdump_sig = (default)        # CONTROL: Sigdump signal (default: disabled)
nonstop = (default)            # BOOT: Nonstop mode
stop_at_load = (default)       # BOOT: Stop at just loading location
init_script = (default)        # BOOT: debug command script path loaded at first stop
commands = (default)           # BOOT: debug commands invoked at first stop. commands should be separated by ';;'
no_rc = (default)              # BOOT: ignore loading ~/.rdbgrc(.rb)
history_file = (default)       # BOOT: history file (default: ~/.rdbg_history)
save_history = (default)       # BOOT: maximum save history lines (default: 10,000)
port = (default)               # REMOTE: TCP/IP remote debugging: port
host = (default)               # REMOTE: TCP/IP remote debugging: host (localhost if not given)
sock_path = (default)          # REMOTE: UNIX Domain Socket remote debugging: socket path
sock_dir = (default)           # REMOTE: UNIX Domain Socket remote debugging: socket directory
cookie = (default)             # REMOTE: Cookie for negotiation
open_frontend = (default)      # REMOTE: frontend used by open command (vscode, chrome, default: rdbg).
chrome_path = (default)        # REMOTE: Platform dependent path of Chrome (For more information, See [here](https://github.com/ruby/debug/pull/334/files#diff-5fc3d0a901379a95bc111b86cf0090b03f857edfd0b99a0c1537e26735698453R55-R64))
parent_on_fork = (default)     # OBSOLETE: Keep debugging parent process on fork (default: false)

After

(rdbg) config    # command
log_level = :WARN                  # UI: Log level same as Logger (default: WARN)
show_src_lines = 10                # UI: Show n lines source code on breakpoint (default: 10)
show_frames = 2                    # UI: Show n frames on breakpoint (default: 2)
use_short_path = false             # UI: Show shorten PATH (like $(Gem)/foo.rb) (default: false)
no_color = false                   # UI: Do not use colorize (default: false)
no_sigint_hook = false             # UI: Do not suspend on SIGINT (default: false)
no_reline = false                  # UI: Do not use Reline library (default: false)
skip_path = nil                    # CONTROL: Skip showing/entering frames for given paths
skip_nosrc = false                 # CONTROL: Skip on no source code lines (default: false)
keep_alloc_site = false            # CONTROL: Keep allocation site and p, pp shows it (default: false)
postmortem = false                 # CONTROL: Enable postmortem debug (default: false)
fork_mode = :both                  # CONTROL: Control which process activates a debugger after fork (both/parent/child) (default: both)
sigdump_sig = false                # CONTROL: Sigdump signal (default: false)
nonstop = false                    # BOOT: Nonstop mode (default: false)
stop_at_load = false               # BOOT: Stop at just loading location (default: false)
init_script = nil                  # BOOT: debug command script path loaded at first stop
commands = nil                     # BOOT: debug commands invoked at first stop. commands should be separated by ';;'
no_rc = false                      # BOOT: ignore loading ~/.rdbgrc(.rb) (default: false)
history_file = "~/.rdbg_history"   # BOOT: history file (default: ~/.rdbg_history)
save_history = 10000               # BOOT: maximum save history lines (default: 10000)
port = nil                         # REMOTE: TCP/IP remote debugging: port
host = "127.0.0.1"                 # REMOTE: TCP/IP remote debugging: host (default: 127.0.0.1)
sock_path = nil                    # REMOTE: UNIX Domain Socket remote debugging: socket path
sock_dir = nil                     # REMOTE: UNIX Domain Socket remote debugging: socket directory
cookie = nil                       # REMOTE: Cookie for negotiation
open_frontend = nil                # REMOTE: frontend used by open command (vscode, chrome, default: rdbg).
# REMOTE: Platform dependent path of Chrome (For more information, See [here](https://github.com/ruby/debug/pull/334/files#diff-5fc3d0a901379a95bc111b86cf0090b03f857edfd0b99a0c1537e267356984
53R55-R64))
chrome_path = nil
parent_on_fork = false             # OBSOLETE: Keep debugging parent process on fork (default: false)
(rdbg)

[st0012]

cc @peterzhu2118

[peterzhu2118]

Looks good, thanks!