Deal with insertRule edge-case.
When there's trailing garbage after an @import rule we throw, but we
still trigger the load (that's not great but not trivial to change).
Deal with that case before calling ImportRuleLoaded().
Differential Revision: https://phabricator.services.mozilla.com/D219783
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1914106
gecko-commit: 2ec3ea09d581a55f400d5d79077a3c18ebbe6978
gecko-reviewers: jwatt