Allow negative utility names in @utilty

[RobinMalfait]

This PR fixes an issue where static utilities defined via @utility wasn't possible if the name starts with -.

There are plenty of static utilities that start with -, but it wasn't possible to register them via the @utility directive, only via the JS API.

Example of a core utility that is now valid:

@utility -inset-full {
  inset: -100%;
}