utility classes: support modifiers like disabled:, hover:, etc.

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From #13 suggested by @owahltinez

Stuff like disabled: and hover: are very handy as well, but I don't know if that's feasible without blowing up all the possible combinations.

Check which modifiers are worth implementing out-of-the-box

I'm not entirely sure whether this should be in matcha scope or not.
If there are enough demand I guess it could be worth to implement.

Also I'm not sure all combinations are equals, feel like most users would use this with colors classes when hovering/focusing and maybe to change opacity/cursors on disabled stuff for example.

So maybe the scope is actually smaller than imagined