[selectors] Add a "case-sensitive" attribute-matcher flag

[tabatkins]

A while back we added a "case-insensitive" flag for attribute-matching, to help model how a number of HTML attributes matched their value case-insensitively.

However, per whatwg/html#4158, the type attribute, at least, always matches case-insensitively, no matter whether you provide the flag or not. (This is a legacy weirdness thing, not an intentional design.) This, then, makes it actually impossible to implement the ol styling rules in CSS; they can only be applied magically or via JS examining the actual value of the attribute.

Perhaps we could add a "case-sensitive" flag that forces matching to be done case-sensitively, even in cases like this where it's case-insensitive by default? @annevk suggests a c, which is as good as anything else.

[Loirooriol]

Dupe of #2101?

[tabatkins]

Indeed, thanks! Closing this in favor of discussion over there.