package : selector specificity

[romainmenke]

I wanted to add more tests and some docs for the specificity calculations.
This was non-trivial when the code was embedded in plugins.

I also looked around again but didn't find any existing package that does this without also having a compromise I do not want to make (like adding another parser)

Extracting this into its own package and publishing this will be best.

---

import parser from 'postcss-selector-parser';
import selectorSpecificity from '@csstools/selector-specificity';

const selectorAST = parser().astSync('#foo:has(> .foo)');
const specificity = selectorSpecificity(selectorAST);

console.log(specificity.a); // 1
console.log(specificity.b); // 1
console.log(specificity.c); // 0

[Antonio-Laguna]

This looks great!

Thanks for all of the tests too!