[css-syntax] Implement recent syntax spec change to avoid parsing custom properties that look like selectors.
This implements the:
If the first two non- values of rule’s prelude are
an whose value starts with "--" followed by a
, then...
From https://drafts.csswg.org/css-syntax/#consume-qualified-rule
Differential Revision: https://phabricator.services.mozilla.com/D207796
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1884879
gecko-commit: cbad005d70dc823e74fd3d85f230de44040ccca8
gecko-reviewers: firefox-style-system-reviewers, zrhoffman, supply-chain-reviewers, glandium