Allow variant to be an at-rule without a prelude

[thecrypticace]

Right now you can't define a custom variant for something like @starting-style which has an empty at-rule prelude.

This PR addresses this shortcoming and now defining the following variant no longer errors:

addVariant('starts-with', '@starting-style')

This change will allow you to use a utility like starts-with:underline to produce the following CSS:

@starting-style {
  .starts-with\:underline {
    text-decoration-line: underline;
  }
}