Add _missing_ implementation for string enumerations that have supportsCustomValues specified in schema.

LSP-json has a code that assigns custom language ID to TextDocument.languageId. That triggers a type error:

Could not assign item in TypedDict. "Literal['jsonc']" is not assignable to "LanguageKind" (reportGeneralTypeIssues)

Adding _missing_ implementation allows us to use a code like:

text_document['languageId'] = LanguageKind('jsonc')

to set custom language ID without triggering type error.