Explicitly ignore --inset-shadow and --inset-ring variables in inset handler

[adamwathan]

Resolves #14440.

This PR fixes an issue where registering a custom inset-shadow-* utility value in your theme like this:

@theme {
  --inset-shadow-potato: inset 0px 2px 6px #7a4724;
}

…mistakenly generates both an inset-shadow-* and inset-* utility with that value:

.inset-shadow-potato {
  inset: inset 0px 2px 6px #7a4724;
}

.inset-shadow-potato {
  box-shadow: inset 0px 2px 6px #7a4724;
}

This replaces #14445 which turns out to not be the ideal solution.

Now we just explicitly ignore variables like --inset-shadow-* and --inset-ring-* in the inset handler. Kind of a gross patch but I can live with it because the whole existence of the --inset-* key is kind of a backwards compatibility thing anyways.