Ensure invalid typehints are not generated

[RobinMalfait]

This PR prevents generating unknown typehints. This is because in a lot of plugins we use the value as-is (any typehint, which is the default).

input:

<div class="inset-[hmm:12px]"></div>

output:

If we didn't have this fix, this would have generated:

output:

.outline-\[hmm\:12px\] {
    outline: hmm: 12px;
}

Which in turn makes the whole css document invalid and it will break your build. No bueno!