[css-typed-om] CSSKeywordValue should serialize using "serialize an identifier"

[AtkinsSJ]

Originally posted here: w3c/csswg-drafts#12545

The spec for this simply says:

Return this’s value internal slot.

However, the internal slot may have spaces or other characters that need escaping. The browsers that implement typed-om all escape these, equivalent to using the serialize an identifier algorithm, as demonstrated by the WPT results for the test that checks CSSKeywordValue serialization.