eslint-plugin: check for invalid selectors

[NotWoods]

This is also checked at runtime but it would be nice to have it in the linter for CI.

makeStyles({
  foo: {
    // invalid, should be '&.withClass'
    '.withClass': {}
  }
})

It might be useful to create a @griffel/validation package to share these checks between core and the eslint-plugin. That could also hold the set of invalid shorthand selectors.