Refactor selector-max-specificity to leverage upstream helper

[romainmenke]

Which issue, if any, is this issue related to?

None

Is there anything in the PR that needs further explanation?

While working on selector-max-specificity for the end position fix I noticed that all of the complexity in the rule is for the ignoreSelectors secondary option.

Because this option is present it is needed to manually walk the selector and re-implement large sections of @csstools/selector-specificity so that ignoring selectors works as expected.

I've added an option upstream to customize the specificity calculations.
See : https://github.com/csstools/postcss-plugins/blob/main/packages/selector-specificity/docs/selector-specificity.selectorspecificity.md

Because this is integrated into @csstools/selector-specificity we can offload a lot more complexity to that package.

A good example was all of the logic around :nth-child(n of .foo).
All this logic already existed upstream but couldn't be used before.

---

A side effect of this change is that this rule is also a bit faster.

Without secondary options.

Main:

node scripts/benchmark-rule.mjs selector-max-specificity '10,10,10'
Warnings: 0
Mean: 370.16875366666665 ms
Deviation: 16.015808005438103 ms

This change:

node scripts/benchmark-rule.mjs selector-max-specificity '10,10,10'
Warnings: 0
Mean: 301.923139 ms
Deviation: 13.039688509741097 ms

With secondary options.

Main:

node scripts/benchmark-rule.mjs selector-max-specificity '["10,10,10", { "ignoreSelectors": "foo" }]'
Warnings: 0
Mean: 373.55708683333336 ms
Deviation: 17.037700655785976 ms

This change:

node scripts/benchmark-rule.mjs selector-max-specificity '["10,10,10", { "ignoreSelectors": "foo" }]'
Warnings: 0
Mean: 317.46817016666665 ms
Deviation: 19.0016953176133 ms

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Latest commit: 1aaa7ff

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[Mouvedia]

A side effect of this change is that this rule is also a bit faster.

Be bold and say that's what you intended all along :)

[ybiquitous]

@romainmenke Thanks for the pull request. This is a great improvement! Overall LGTM, but I've left a few comments, so I'd appreciate if you would check them out.

[ybiquitous]

Thanks, LGTM 👍🏼

[romainmenke]

Thank you for the reviews 🙇