Don't prefix classes in arbitrary variants

[thecrypticace]

We were incorrectly prefixing classes in arbitrary variants if the first variant wasn't an arbitrary variant. Additionally, if the first variant was arbitrary then all classes in all variants would end up un-prefixed. For example:

<div class="[.foo_&]:group-hover:tw-underline">test</div>
<div class="group-hover:[.foo_&]:tw-underline">test</div>

These would result in the selectors:

.foo .group:hover .\[\.foo_\&\]\:group-hover\:tw-underline
.tw-group:hover .tw-foo .group-hover\:\[\.foo_\&\]\:tw-underline

Notice how:

1. The first one doesn't have the group class prefixed like it should.
2. The second has the foo class prefixed when it should not.

Here we rework the functions finalizeSelector and formatVariantSelector together to take multiple formats. They were previously using a mix for AST and string-based parsing. This now does the full transformation using the selector AST, parses the format strings AST as early as possible, and is set up to parse them only once for a given set of rules. All of this will allow considering metadata on a per format string basis. For instance, we now know if the format string .foo & was produced by a normal variant or by an arbitrary variant. We use this information to control the prefixing behavior for individual format strings.

Fixes #10125

[RobinMalfait]

Looks good! Pretty nice that we didn't have to rewrite everything to handle the isArbitraryVariant per collected format.