[css-properties-values-api] Define parsing rules for the syntax string. (#860)

* [css-properties-values-api] Define parsing rules for the syntax string.

This defines how to parse the syntax strings using some algorithms and
concepts from css-syntax, while leaning as little as possible on its
concept of tokens.

* Whitespace behavior is now defined.
* Escaping is now allowed for ident syntax strings.

Resolves #112.

* fixups

changes:

* refer directly to algorithms dfns
* use and link to some Infra and Syntax concepts
* strip whitespace from the syntax string
* simplify algorithms a little
* fix a reconsume bug in "consume a syntax component"

Author: andruud