Keep install as the default command and print how to show the help on $ bundle

[eregon]

• Fixes Keep bundle as bundle install by default #9124.

What was the end-user or developer problem that led to this PR?

Changing the default command, even if done in a future release is massive breaking change, and only benefits a few users which expect $ bundle to show the help, as described in #9124.
Asking everyone to configure things is very annoying and leads to "it works on my machine but not on yours".

To avoid the breakage and satisfy what both groups of users want, we print a message on $ bundle for how to show the help.

What is your fix for the problem, implemented in this PR?

I print a message on $ bundle for how to show the help.
Example:

$ cat Gemfile
source 'https://rubygems.org'
gem "path"
group :dev do
  gem "rubocop"
end

$ bundle 
Running `bundle` without argument runs `bundle install`.
If you want the help, use `bundle --help`.
Fetching gem metadata from https://rubygems.org/...........
Resolving dependencies...
Fetching ast 2.4.3
Fetching lint_roller 1.1.0
Fetching json 2.16.0
Fetching language_server-protocol 3.17.0.5
Fetching parallel 1.27.0
Fetching racc 1.8.1
Fetching path 2.1.0
Fetching prism 1.6.0
Fetching rainbow 3.1.1
Fetching regexp_parser 2.11.3
Fetching ruby-progressbar 1.13.0
Fetching unicode-emoji 4.1.0
Installing lint_roller 1.1.0
Installing ast 2.4.3
Installing json 2.16.0 with native extensions
Installing language_server-protocol 3.17.0.5
Installing parallel 1.27.0
Installing racc 1.8.1 with native extensions
Installing path 2.1.0
Installing rainbow 3.1.1
Installing regexp_parser 2.11.3
Installing ruby-progressbar 1.13.0
Installing unicode-emoji 4.1.0
Installing prism 1.6.0 with native extensions
Fetching unicode-display_width 3.2.0
Installing unicode-display_width 3.2.0
Fetching parser 3.3.10.0
Installing parser 3.3.10.0
Fetching rubocop-ast 1.48.0
Installing rubocop-ast 1.48.0
Fetching rubocop 1.81.7
Installing rubocop 1.81.7
Bundle complete! 2 Gemfile dependencies, 17 gems now installed.
Bundled gems are installed into `./vendor/bundle`

The extra 2 lines seems a good compromise to handle the expectations from both groups of users.

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• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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