Expose attr-value types and how to cite in /xref

[marcoscaceres]

For example, <link rel=manifest> is exported from html, and should be citable as [^link/rel/manifest^].

Similarly, we want:

[^html-global/role/button^]

[sidvishnoi]

This works already: https://respec.org/xref/?term=text&specs=html&types=attr-value, but the terms aren't in webref (yet?).

[marcoscaceres]

Weird... I was expecting to see https://respec.org/xref/?term=manifest there as a attribute value for a <link rel=>

[marcoscaceres]

Ok, yeah... they didn't get exported from HTML.