[fonts] Font Provider API changes based on API review

- Introduce tables for existing structs
- Improve organization of definitions; add detailed documentation
- Place all new definitions in provider.fidl (and styles.fidl)
- Incorporate other changes and suggestions from
  https://fuchsia-review.googlesource.com/c/fuchsia/+/249522/

Implementation updates will be completed in subsequent CLs.

I18N-12 #comment

Change-Id: I177f5850b1c9eb91f59af1885005336121c0f1e0