This PR fixes an issue where we didn't always correctly validated
invalid CSS variables when using the CSS variable shorthand syntax.
This PR fixes that by ensuring we start with `--`. We can't validate
that the variable exists at runtime, but we can validate if it looks
like a valid CSS variable.
We can go a step further by validating if the CSS variable is valid
syntax (e.g.: all characters are valid), but we can introduce this if
this is causing actual issues in projects.
```
p-(a-b)
```
Used to compile to:
```css
.p-\(a-b\) {
padding: var(a-b);
}
```
But not anymore.
193fc60989