Add support for theme family-specific syntax mapping overrides (#3551)

This PR adds support for adding a specific set of mappings from Zed
syntax tokens to VS Code scopes for a particular theme family.

We can use this as a fallback when we aren't otherwise able to rely on
the mappings in the theme importer, as sometimes it isn't possible to
make a specific enough matcher that works across all of the themes.

Release Notes:

- N/A