[css-values] add 'revert' and 'revert-layer' to list of CSS-wide keywords

[dbaron]

css-values lists the CSS-wide keywords. This definition is important because it has implications for things like what is allowed in custom identifiers.

css-cascade-4 adds the 'revert' keyword and css-cascade-5 adds the 'revert-layer' keyword. Should these be added to the list of CSS-wide keywords?

I'd note that Blink already mostly implements this change (modulo a bug).

I haven't checked other implementations.

[tabatkins]

Yes, they should be.

[yisibl]

I have a similar issue: #7170