[css-properties-values-api] Add <string> as a supported syntax name

[lilles]

We should add "<string>" as a valid syntax for custom property registration:

https://www.w3.org/TR/css-properties-values-api-1/#supported-names

Chrome got a bug report that "<string>" was expected to work for registered custom properties to be used in the content property.

[lilles]

@andruud @tabatkins does that sound good?

[tabatkins]

Oh, huh, that's a weird omission. Yes, absolutely we should allow strings.

[SebastianZ]

For reference, Mozilla already added parsing <string> today.

Sebastian