[pigeon] Tidy GeneratorAdapters to be const, with getters instead of fields

[srawlins]

While walking through the GeneratorAdapter code, I noticed a few oddities. While tidying things up, I found that each of the subclasses could have const constructors, which I think are preferred by the Flutter team.

• The parent class, GeneratorAdapter, had a fileTypeList field which never stored a value. Each subclass implements the parent class, so the constructor was never called. I changed the field to a getter (for the interface), and removed the constructor.
• Some adapters had a mutable instance field, String languageString which could be made static and const.
• In each adapter subclass, I made the List<FileType> fileTypeList into a getter, to avoid the "Fields in const classes should not have initializers" enforced lint rule.
• Some of the adapter subclasses had this.fileTypeList as a constructor parameter, but a value was essentially never passed in. (There was once instance in a test, but it had no effect.) So I removed this constructor parameter, moving the const default values to the getter value.
• Then each constructor can be made const.
• The implementation of _openSink had some unnecessary statements, so I made it more concise. The sink variable was unnecessary, and the file variable was only used in one else-branch, and so should be declared in that block.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request refactors the GeneratorAdapter classes to enable const constructors. This involves converting the fileTypeList instance field to a getter in GeneratorAdapter and its subclasses, and changing languageString to a static const field. Additionally, the _openSink function is refactored for conciseness by removing unnecessary local variables.

[tarrinneal]

Seems like this is old code that I would guess was just here for testing when it was added. This seems like a good improvement, thank you

[tarrinneal]

@stuartmorgan-g more exceptions here I believe

[stuartmorgan-g]

This is a non-trivial (even if relatively straightforward) set of changes to production code, so our standard practice to version this for continuous release applies.

[srawlins]

Thanks! I've bumped the version now.