[Gecko Bug 1884879] [css-syntax] Implement recent syntax spec change to avoid parsing custom properties that look like selectors.

[moz-wptsync-bot]

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