Add meta schema support for enum types

[jack-berg]

@marcalff's comment here made me realize that meta schema support for enum types is bad, preventing us from describing each individual enum value, and from setting language implementation support status for each individual enum value.

In case of translation_strategy for prometheus, I have a case where not all enum values are supported.

This improves it.

• Each type in meta_schema_types.yaml now has either .properties or .enumValues based on whether its an object or enum type
• Each type in meta_schema_languages_{language}.yaml now has either .propertyOverrides or .enumOverrides based on whether its an object or enum type
• schema-docs.md now recognizes the enum types from object types, and displays enum types better, for example:

[codeboten]

This makes sense to me

[marcalff]

LGTM