Ensure -- is allowed inside candidates

[RobinMalfait]

This PR fixes an issue where named utilities that contain double dashes -- are not extracted correctly.

Some people use -- in the middle of the utility to create some form of namespaced utility.

Given this input:

let x = 'foo--bar'

The extracted candidates before this change:

[ "let", "x", "--bar" ]

The extracted candidates after this change:

[ "let", "x", "foo--bar", "--bar" ]

The reason --bar is still extracted in both cases is because of the CSS variable machine. We could improve its extraction by checking its boundary characters but that's a different issue.

For now, the important thing is that foo--bar was extracted.

Test plan

1. Added new test
2. Existing tests pass