Prompt for default CLI command when running bundle command

[hsbt]

This PR modifies Bundler's behavior when running bundle without any arguments.

The changes introduce an interactive prompt that asks users to choose their preferred default command, rather than automatically running bundle install with just a deprecation warning.

[eregon]

I think this does not address #9124.
Having a prompt on $ bundle is unexpected, and e.g. if bundle install takes a while, and the user focuses on something else in the meanwhile, and then they come back to see $ bundle has done nothing, they could be pretty annoyed.
Also if the default would be switched in a future version then this would still break CI where it's not possible to prompt.

[kou]

How about also adding BUNDLE_DEFAULT_COMMAND=install to avoid the first prompt?

If a user wants to keep using bundle as bundle install, they can use export BUNDLE_DEFAULT_COMMAND=install; bundle with Bundler < 4 and >= 4.