Add macro metadata dart_model types and converter.

[davidmorgan]

• Two generate scripts: generate_definitions which generates dart_model definitions from _fe_analyzer_shared code, and generate_converter which generates code to convert from _fe_analyzer_shared code to the new dart_model types.
  • Neither is intended to be maintained, I'll delete if we don't need them again or rewrite if we need to iterate / use them more than once.
• Add definitions from generate_definitions to macro_metadata_definitions.dart.
• Add package _analyzer_cfe_macros for common code between _analyzer_macros and _cfe_macros, this code will likely end up in _fe_analyzer_shared.
• Add converter from generate_converter to _analyzer_cfe_macros and simple unit test.