Add options for handling `[extras]` dependencies

[devmotion]

This PR extends #502 based on @DilumAluthge's suggestions in https://github.com/JuliaRegistries/CompatHelper.jl/pull/502#issuecomment-3183867774.

I added an option open_prs_for_extras that can be set to IfExistingCompatExtras() (the default), AllExtras() or NoExtras() [my gut feeling is that the names could be improved a bit]. With IfExistingCompatExtras(), only [extras] dependencies with existing compat entry are updated; with AllExtras(), all [extras] dependencies are updated (also those without existing compat entry); and with NoExtras(), [extras] dependencies are not updated.

I went with singleton types (basically a type-based enum) because 1) it felt more consistent with the existing EntryType and 2) it's not necessary to check whether the provided value is valid (which would be necessary if the API would be based on Symbols).

Fixes #166. Closes #502.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 100.00%. Comparing base (7ea4330) to head (cd36c36). :warning: Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #528   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        11    -1     
  Lines          412       411    -1     
=========================================
- Hits           412       411    -1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[DilumAluthge]

Could we make this experimental for now? That way, we can try it out, and if we don't like the API, we can change it?

[DilumAluthge]

By "experimental", I mean explicitly document that we are allowed to break it in a minor (or even patch) release of CompatHelper.

[devmotion]

I added a note.

[devmotion]

@DilumAluthge this PR is ready for another round of review 🙂

[devmotion]

@DilumAluthge bump 🙂

[DilumAluthge]

I can try to review this weekend.

[DilumAluthge]

I can try to review this weekend.

Famous last words, huh?

[devmotion]

@DilumAluthge can you create a new release?

[DilumAluthge]

https://github.com/JuliaRegistries/CompatHelper.jl/pull/536