migrate `TargetKind` enum to classes

[pq]

Follow-up from: #49796. This is required to evolve support for existing and upcoming language features and will unblock:

• TargetKind has no value for representing a type parameter #49796
• Support mustBeOverriden on constructors #53357
• Add a TargetKind for annotations on imports, exports, and part directives. #52274
• [meta] Add TargetKind.constructor #47421

Background: an earlier attempt at this refactoring was reverted in 813ca7f for appearing to cause a measurable performance regression.

813339e is a trimmed down version of that attempted change; with it landed, we'll monitor any performance impact. If there is no impact we'll publish a new version of package:meta (bumping minor version) and see if any unexpected ecosystem impact shakes out.

[pq]

UPDATE package:meta v1.10.0 with this change has been published. Assuming no ecosystem issues shake out, we'll go ahead and start adding new TargetKinds.

[pq]

8a5ddec adds the first new target kind (for extension types). We haven't published that yet so we can still back it out but it looks like this migration may be in the clear. 🤞

[pq]

This feels done. If we see any issues we can reopen.