Generator fixes, ignore lint

• Fix generation support for optional generic bounds, e.g.
  class Foo<T extends Object?>.
• Fix generation for classes with names starting $.
• Ignore lint unnecessary_lambdas in generated code.

Generator improvements

• Change generated build methods to return only public types, creating
  _build methods that return the generated impl types. This means dartdoc
  will no longer reference the generated types.
• Ignore the no_leading_underscores_for_local_identifiers lint in generated
  code.
• Migrated built_value_generator to null safety. This is purely an internal
  change, the generator can still generate legacy code as and when needed.

Fix serializer generation corner case

• Bug fix: fix a corner case with generics that had incorrect serializer generation.

Fix build warning

• Bug fix: remove a print from the enum generator.

Fix deps

• Fix deps: allow built_value_generator to use built_value 8.2.0.

Generator improvements for lints, bump `analyzer`

• Allow writing final parameters in EnumClass constructor and valueOf method.
• Make generator output additional explicit null checks so the generated code complies with the cast_nullable_to_non_nullable lint.
• Bump version of analyzer.

Bump version of `analyzer`

• Bump version of analyzer.

Bump version of `analyzer`

• Bump version of analyzer, fix deprecation warnings.

Bump version of `analyzer`

• Bump version of analyzer.

Bug fix for constructor annotations

• Bug fix: allow constructors to have annotations. Previously, annotations
  would cause codegen to fail.