Auto merge of #14535 - servo:property-id, r=mbrubeck

Introduce a PropertyId enum and use it instead of strings of property names

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* `LonghandId` and `ShorthandId` are C-like enums
* `Atom` is used for the name of custom properties.
* `PropertyDeclarationId` is the identifier for `PropertyDeclaration`,
  after parsing and shorthand expansion. (Longhand or custom property.)
* `PropertyId` represents any CSS property, e.g. in CSSOM.
  (Longhand, shorthand, or custom.)

CC @upsuper

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- [ ] These changes fix #__ (github issue number if applicable).

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