[cssom] Add supportsText to StyleSheet and handle HTML <link media="supports()">

[zcorpan]

In whatwg/html#7658 by @mirisuzanne the media attribute for <link rel=stylesheet> is changed to take an import condition (rather than a media query list), but I think that requires changes to CSSOM to work:

For <link rel=stylesheet media=...> the media attribute is used in the CSSOM "create a CSS stylesheet" steps and a MediaList object gets created from the attribute's value, where that vaule is parsed as a media query list.

Meanwhile, for @import, the CSSImportRule interface has a supportsText attribute for the <supports-condition> part.

I think CSSOM needs to change to allow creating stylesheets with supports conditions, and changing the HTML media attribute needs to update both the MediaList's mediaText and the StyleSheet's supportsText. I'll file an issue for CSSOM.

(Maybe also CSSStyleSheetInit (options for constructable stylesheets) should support supports, but that's not blocking this PR.)

[zcorpan]

I guess this is a dup of #9087.