[Extension type] Analyzer implementation

[itsjustkevin]

Updated: opened bugs related to extension types:

• Not regular-bound extension type crashes compiler #53123
• co19/LanguageFeatures/Extension-types/static_analysis_extension_types_A24_t02 fails in analyzer #53206
• Failures on Extension types. Report when await of not Future subtype. #53219
• Analyzer: static warning support for extension types #53434
• Representation variable of an extension type is not promoted #53446
• [analyzer] No error if an extension type declares getter and setter with the same basename but with uncompatible types #53489
• [Extension types] diagnostic when implementing a class with a conflicting member #53567
• [extension types] Extension type does not get a highlight in DartDoc #53592
• [analyzer] [extension types] Implement 'precludes' rule to eliminate method/setter conflicts #53719
• Analyzer reports invalid matches for valid type checks on extension types #53725
• CFE and analyzer: constant error when casting null to nullable extension type. #53751
• [Analyzer] Error when calculating upper bound of an extension type #53781
• [analyzer][Extension types] No error when extension type has a member conflict #53920
• [Extension types] Analyzer reports incorrect "const eval throws" for extension constructor. #53935
• Support for extension types in strict modes #53982
• [extension type] Analyzer has incorrect constant identity for type literals of nested extension types. #54057
• [Extension type] Implement new nullaility rules in analyzer #54079
• [Extension types] No error in analyzer if result of == operator is assigned to not bool #54080
• [Analyzer] Improve error message when extension type is not well-bounded #54098

[srawlins]

Still blocked on dart-lang/language#2420

[eernstg]

Please note that this feature has been renamed: It is now the 'inline class' feature.

[pq]

And here's the spec: inline-classes/feature-specification.md.

[srawlins]

Closing as more-or-less complete; the last task is tracked with it's own issue.