Fix error in internally tagged enums with flattened fields

[gustavo-shigueo]

Goal

Fix internally tagged enum with struct variant containing inlined field
Closes #343

Changes

Instead of attempting to format the variant in the format_variant function, allow type_def to generate a fully formatted variant by giving it the variant's name and tag

Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.

[gustavo-shigueo]

This also fixes an oversight in #244 by wrapping the name of struct tag fields in quotes, like already happened in enums. This allows a tag to be any string, rather than only valid JS identifiers, e.g.: kebab-case string