Analyzer: static warning support for extension types

[srawlins]

warnings based on meta annotations:

• @experimental, @internal, @protected, @visibleForTesting - does it work on a method declared in an extension type? does it work on a method "implemented" (exposed) via an extension type? If an extension type itself is annotated with any of these, does enforcement work when referencing the extension type? Does enforcement work when referencing/calling the primary constructor?
  • @experimental (@pq)
  • @internal (@pq)
  • @protected (@pq)
  • @visibleForTesting (@pq)
• @factory (@pq)
• @immutable (@pq)
• @literal - (@pq)
• @mustBeOverridden - should be disallowed on methods declared in extension types (and extensions if not already) (@pq)
• @mustCallSuper - should be disallowed on methods declared in extension types (and extensions if not already) (@pq)
• @nonVirtual - should be disallowed on methods declared in extension types (and extensions if not already) (@pq)
• @optionalTypeArgs - can annotate a type extension (@pq 8a5ddec)
• @reopen - cannot annotate a type extension (or an extension, if not already banned) (@pq)
• @sealed - cannot annotate a type extension (or an extension, if not already banned) (@pq)
• @visibleForOverriding - cannot annotate a type extension or extension type member (@pq)

other warnings:

• DEPRECATED_MEMBER_USE - test against extension type, member declared in extension type, member "implemented" (exposed) by extension type
• DIVISION_OPTIMIZATION - what if x or y are an extension type on int?
• INFERENCE_FAILURE_ON_* - I think tests are needed both for expressions passed to extension type constructor, and extension typed expressions passed elsewhere
• OVERRIDE_ON_NON_OVERRIDING_* - for anything declared in an extension type
• RETURN_OF_INVALID_TYPE_FROM_CATCH_ERROR and RETURN_TYPE_INVALID_FOR_CATCH_ERROR - return type being an extension type on Future should be ok.
• STRICT_RAW_TYPE - should be reported for an extension type missing type arguments. (@pq)
• UNNECESSARY_TYPE_CHECK (@pq)
• UNUSED_ELEMENT - [Extension Types] analyzer should produce UNUSED_ELEMENT diagnostics for unused extension type decls #53520 (@pq)

[pq]

Happy to take a look at (some bits of) this.

Some annotation support will get ungated by #53368