Allow trailing dash in functional utility names (#19696)

## Problem

Tailwind 4.2.0 introduced stricter `@utility` name validation (#19524)
that rejects functional utility names where the root ends with a dash
after stripping the `-*` suffix. This breaks a valid and useful naming
pattern where a double dash separates the CSS property from a value
scale:

```css
@utility border--* {
  border-color: --value(--color-border-*, [color]);
}
```

This produces: `border--0`, `border--1`, `border--2`, etc.

The error message is:

> `@utility border--*` defines an invalid utility name. Utilities should
be alphanumeric and start with a lowercase letter.

## Why this pattern matters

The double-dash convention creates a clear visual grammar in class
names. The first segment names the CSS property, and the double dash
separates it from the semantic scale value. In a dense className string
like `border border--0 background--0 content--4`, the scale values (0,
0, 4) are immediately scannable, distinct from the single-dash property
names around them.

This pattern is actively used in production design systems for semantic
color scales (background, content, border, shadow) with values from
0-10.

## Why the restriction is unnecessary

The validation comment states the concern is that `border--*` could
match the bare class `border-` when using default values. However, this
edge case is already handled:

1. **`findRoots` in `candidate.ts`** (line 887) already rejects empty
values: `if (root[1] === '') break`
2. **The Oxide scanner** already extracts double-dash candidates
correctly, as confirmed by existing tests: `("items--center",
vec!["items--center"])`

The candidate parser and scanner both handle this case. The validation
was an overcorrection.

## Changes

- Removed the trailing-dash check from `isValidFunctionalUtilityName` in
`utilities.ts`
- Updated the existing unit test from `['foo--*', false]` to `['foo--*',
true]`
- Added an integration test proving `@utility border--*` compiles
correctly with theme values

## Test results

All 4121 tests pass across the tailwindcss package, including the new
integration test.

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>