[cssom] Handle import conditions instead of media queries

[mirisuzanne]

The larger goal here is to allow authors to apply new css features (like cascade layers) while linking stylesheets. In order to do that, we've defined an import condition production which can be used in both CSS @import and the HTML media attribute.

As Domenic points out:

• https://drafts.csswg.org/cssom/#concept-css-style-sheet-media does not need changes but will start being given import condition strings instead of media query strings by HTML. That leads to changes to the following sections:
• https://drafts.csswg.org/cssom/#media-queries needs to handle import conditions instead of media queries. (But, we cannot change the names MediaList and mediaText.)

/cc @tabatkins