Re-introduce automatic var injection shorthand (#15020)

This PR re-introduces the automatic var injection feature.

For some backstory, we used to support classes such as `bg-[--my-color]`
that resolved as-if you wrote `bg-[var(--my-color)]`.

The is issue is that some newer CSS properties accepts dashed-idents
(without the `var(…)`). This means that some properties accept
`view-timeline-name: --my-name;` (see:
https://developer.mozilla.org/en-US/docs/Web/CSS/view-timeline-name).

To make this a tiny bit worse, these properties _also_ accept
`var(--my-name-reference)` where the variable `--my-name-reference`
could reference a dashed-ident such as `--my-name`.

This makes the `bg-[--my-color]` ambiguous because we don't know if you
want `var(--my-color)` or `--my-color`.

With this PR, we bring back the automatic var injection feature as
syntactic sugar, but we use a different syntax to avoid the ambiguity.
Instead of `bg-[--my-color]`, you can now write `bg-(--my-color)` to get
the same effect as `bg-[var(--my-color)]`.

This also applies to modifiers, so `bg-red-500/[var(--my-opacity)]` can
be written as `bg-red-500/(--my-opacity)`. To go full circle, you can
rewrite `bg-[var(--my-color)]/[var(--my-opacity)]` as
`bg-(--my-color)/(--my-opacity)`.

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This is implemented as syntactical sugar at the parsing stage and
handled when re-printing. Internally the system (and every plugin) still
see the proper `var(--my-color)` value.

Since this is also handled during printing of the candidate, codemods
don't need to be changed but they will provide the newly updated syntax.

E.g.: running this on the Catalyst codebase, you'll now see changes like
this:
<img width="542" alt="image"
src="https://github.com/user-attachments/assets/8f0e26f8-f4c9-4cdc-9f28-52307c38610e">

Whereas before we converted this to the much longer
`min-w-[var(--button-width)]`.

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Additionally, this required some changes to the Oxide scanner to make
sure that `(` and `)` are valid characters for arbitrary-like values.

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Co-authored-by: Adam Wathan <adam.wathan@gmail.com>