Improve parsing of shadow values (#20080)

This PR fixes an issue where a `calc(…)` value used in a shadow value
was incorrectly marked as the color of that shadow.

We have this feature where we can have colored shadows, this requires us
to replace the color in a value with a `var(--tw-shadow-color,
<original-value-here>)` such that we can swap out the color.
For this, we have to parse the value and figure out what the color part.

This PR uses the `ValueParser` to parse values, then figure out what the
color part is. The biggest reason for this is that we know what a
"function" is and what a normal "word" is, which allows us to do more
fine-grained checks on a per-type basis.
We tracked a few more functions that we _know_ produce color values, and
a few cases that we know produce length values so therefore can't be the
color.

Some examples:

- `calc(…)`, `min(…)`, `max(…)`, `clamp(…)`, `--spacing(…)` — these all
produce length-values.
- `color(…)`, `color-mix(…)`, `rgba?(…)`, `--alpha(…)` … — these all
produce color values.

Notice that we added `--spacing(…)` and `--alpha(…)` as well, these are
custom functions that Tailwind CSS provides, but we know what it will
eventually map to internally.

Last but not least, we also detect named colors and hex-based colors.

Fixes: #20065 
Closes: #20074

## Test plan

1. Added an integration test, before the fix the test would've looked
lik this:

  ```diff
    .drop-shadow-calc {
- --tw-drop-shadow-size: drop-shadow(0 0 calc(1 * var(--spacing))
var(--tw-drop-shadow-color, black));
+ --tw-drop-shadow-size: drop-shadow(0 0 var(--tw-drop-shadow-color,
calc(1 * var(--spacing))) black);
      --tw-drop-shadow: drop-shadow(var(--drop-shadow-calc));
filter: var(--tw-blur, ) var(--tw-brightness, ) var(--tw-contrast, )
var(--tw-grayscale, ) var(--tw-hue-rotate, ) var(--tw-invert, )
var(--tw-saturate, ) var(--tw-sepia, ) var(--tw-drop-shadow, );
    }
  ```

2. Added more tests related to the shadow replacement logic itself where
we try to infer the color (or the length-values for x, y, blur, spread)
3. All other existing tests pass