Adds CSS abstract types to existing property features

[vwallen]

Split off from existing PR: #2111

Keys related to abstract types are distributed to existing property features most associated with that type, where that would not change the support status of the feature

A few orphan keys remain with no associated properties:

  - css.types.frequency
  - css.types.frequency-percentage
  - css.types.frequency.hz
  - css.types.frequency.khz
  - css.types.time-percentage

[jamesnw]

This distribution makes sense to me.

For the remaining keys-

• Frequency, could they be their own feature?
• time-percentage- could that go in to animations-css with time?

Alternatively, should they get added to some sort of CSS catchall? Or removed from BCD, since they aren't used at all?

@ddbeck What should we do about the keys that are in BCD, and in the spec, but are not actually usable anywhere?

[ddbeck]

@ddbeck What should we do about the keys that are in BCD, and in the spec, but are not actually usable anywhere?

I've opened mdn/browser-compat-data#24969 to remove the <frequency> and <frequency-percentage> types. I'm very confident they do not belong in BCD, at all.

That leaves <time-percentage>. Is that also not used in any actual properties? If so, I'd be happy to open a PR against BCD to remove that too.

[ddbeck]

This all looks right to me. If <time-percentage> is definitely unused, then this is ready to merge. Thank you!

[jamesnw]

@ddbeck Victor and I have not been able to find any references to time-percentage in the specs, and can't find any use cases. I think it can be removed as well.