Sync css.types.attr with CSS Values 5 extensions (#26624)

Changes:
- Renamed `fallback` to `declaration-value`.
- Renamed `type-or-unit` to `type_function`, adding subfeatures: `<custom-ident>`, `<ident>`, `<image>`, `<length-percentage>`, `<resolution>`, `<string>`, `<transform-function>`.
- Wrapped all keywords in fences.
- Added MDN urls, and spec urls.

Author: caugner