Escape autocompletion word before putting it into Regexp

[st0012]

Fixes #497

The issue is caused by special characters be converted into the autocompletion's RegExp directly, like /\Aargs[/.
It can be reproduced by typing args[ and tab to trigger the completion callback:

❯ exe/rdbg target.rb
[1, 1] in target.rb
=>   1| a = 1
=>#0    <main> at target.rb:1
(rdbg) args[#<Thread:0x00007fa74da10c60@DEBUGGER__::SESSION@server /Users/st0012/projects/debug/lib/debug/session.rb:147 run> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
        29: from /Users/st0012/projects/debug/lib/debug/session.rb:167:in `block in activate'
        28: from /Users/st0012/projects/debug/lib/debug/session.rb:197:in `session_server_main'
        27: from /Users/st0012/projects/debug/lib/debug/session.rb:253:in `process_event'
        26: from /Users/st0012/projects/debug/lib/debug/session.rb:331:in `wait_command_loop'
        25: from /Users/st0012/projects/debug/lib/debug/session.rb:331:in `loop'
        .....