[analyzer/meta] Proposal: Work towards package:meta next major release

[parlough]

There are a few annotations that exist in package:meta that don't make sense in either a Dart 2 or Dart 3 world. To reduce API surface area and potentially misleading or confusing suggestions, work towards the next breaking major release (3.x.x) should be made.

To reduce any potential impact on the ecosystem which is used quite widely, a three step plan similar to the following could be used to get there:

1. Prepare for deprecation

• Keep the ^2.12.0 SDK constraint for now
• Indicate through API comments and changelog expected deprecations, to give another chance to adapt.
• Make a 1.x release with these changes, likely 1.12.0 unless a new release is made before this point.

https://dart-review.googlesource.com/c/sdk/+/342281

2. Increase SDK constraint and deprecate

• Increase SDK constraint to ^3.0.0 as language versions before 2.12 are no longer supported at that point.
• Deprecate @sealed (Deprecate @sealed annotation #53310)
  • If the other uses of the annotation are desired to be maintained, it should likely be a new annotation for clarity (Explain how to write mocks for classes using the new class modifiers site-www#5143 (comment))
• Deprecate @required and @Required (Should pkg/meta's @required be deprecated for Dart v3? #49568)
  • If it's desired to add an annotation that indicates an argument will be required in the future, it should likely be a new annotation (Analyzer, treat @required in null-safe code as a deprecation notice. #49066). This could be part of this release if determined as essential.
• Make any other desired deprecations, such as any annotations currently marked as experimental.
  • An example: Do consider deprecating the meta_dart2js annotations if they aren't providing the value they hoped for.
• Make a final 1.x release, perhaps will be 1.13.0 with these deprecations.

3. Update to next major version and begin implementing breaking changes

• Update pubspec version to 3.0.0-wip to begin work on the breaking changes.
  • 3.0.0 is needed instead of 2.0.0 due to an accidental release of 2.0.0 previously.
• Remove all deprecated annotations
  • This currently includes: @alwaysThrows, @checked, @virtual, and likely @required, @Required, @sealed.
• Mark all classes as final as the classes in package:meta are not intended to be subclassed.
• Clearly document all breaking changes in the package in the changelog.
• Publish the major release :)