Turn --ext option of bundle gem into string. Deprecate usage without explicit value.

[simi]

• this is preparation for onboarding Rust based extension gem generator
• trying to keep it compatible with --ext as much as possible
• partially extracted from https://github.com/rubygems/rubygems/compare/cargo-newgem
• first part of finishing Add support for bundle gem --rust command #5613

Personally I used to create gems with bundle gem --ext hello. Now it prints deprecation when extension type is not provided explicitly, but continues with C extension.

[retro@retro  bundler (ext-enum %=)]❤ ./exe/bundle gem --ext hello
[DEPRECATED] Option `--ext` without explicit value is deprecated. Please pass value like `--ext=c` for C extension. Pretending `--ext=c` was used for now.
Creating gem 'hello'...
MIT License enabled in config
Initializing git repo in /home/retro/code/work/oss/rubygems/bundler/hello
      create  hello/Gemfile
      create  hello/lib/hello.rb
      create  hello/lib/hello/version.rb
      create  hello/sig/hello.rbs
      create  hello/hello.gemspec
      create  hello/Rakefile
      create  hello/README.md
      create  hello/bin/console
      create  hello/bin/setup
      create  hello/.gitignore
      create  hello/test/test_helper.rb
      create  hello/test/test_hello.rb
      create  hello/LICENSE.txt
      create  hello/ext/hello/extconf.rb
      create  hello/ext/hello/hello.h
      create  hello/ext/hello/hello.c
Gem 'hello' was successfully created. For more information on making a RubyGem visit https://bundler.io/guides/creating_gem.html

[simi]

I'll appreciate any suggestion of better text in here. pray

[indirect]

Suggested change

	SharedHelpers.major_deprecation 2, "Option `--ext` without explicit value is deprecated. Please pass value like `--ext=c` for C extension. Pretending `--ext=c` was used for now."
	SharedHelpers.major_deprecation 2, "Extensions can now be generated using C or Rust, so `--ext` with no arguments has been deprecated. Please select a language, e.g. `--ext=rust` to generate a Rust extension. This gem will now be generated as if `--ext=c` was used."

[simi]

I'll use this for the final one (other PR onboarding Rust gem generator).

[simi]

🤔 locally bin/ronn --warnings --roff --pipe --date 2022-12-17 lib/bundler/man/bundle-gem.1.ronn > lib/bundler/man/bundle-gem.1 makes no difference

[indirect]

Perhaps this branch needs to be rebased? GitHub says it is out of date.