Remove automatic var(…) injection

[RobinMalfait]

This PR removes automatic var(…) injection for arbitrary properties, values and modifiers.

There are a few properties that use "dashed-ident" values, this means
that you can use --my-thing as-is without the var(…) around it.

E.g.:

.foo {
  /* Notice that these don't have `var(…)` */
  view-timeline-name: --timeline-name; 
  anchor-name: --sidebar;
}

This causes issues because we are now injecting a var(…) where it's not needed.

One potential solution is to create a list of properties where dashed idents can be used. However, they can also use CSS custom properties that point to another dashed ident.

E.g.:

.foo {
  --target: --sidebar;
  anchor-name: var(--target);
}

A workaround that some people used is adding a _ in front of the variable: mt-[_--my-thing] this way we don't automatically inject the var(…) around it. This is a workaround and gross.

While the idea of automatic var injection is neat, this causes more trouble than it's worth. Adding var(…) explicitly is better.

A side effect of this is that we can simplify the internals for the candidate data structure because we don't need to track dashedIdent separately anymore.

[adamwathan]

Nice, think this is probably the right move. Before we merge this though we should prove we can write a codemod to upgrade people, maybe can do that early next cycle.

[RobinMalfait]

@adamwathan yep, I already added this to our list of breaking changes so we don't forget. I'm sure there are more things we can write codemods for.