CSSJanus: Account for attribute selectors in brace lookahead

Recognize selectors of the forms [attr=val], [attr*=val], [attr~=val]
and [attr^=val], optionally with single-quoted values.

Because these selectors previously weren't recognized, trying to
apply /* @noflip */ to one of them would be recognized as a
single-property noflip rather than a rule-wide noflip, and so only
the first property in the rule would be noflipped.

The (simplified) input that triggered this bug was:

/* @noflip */ figure[typeof*='mw:Image'].mw-halign-left { clear: left; float: left; }

which became { clear: left; float: right; } when run through CSSJanus.

See also I4cddce80397d8 which is a workaround for this issue

Bug: 50910
Change-Id: If424a1df26bb7a5a18cee4b0318b029392528fc0
Merge "CSSJanus: Account for attribute selectors in brace lookahead"