chore: Update @types/csso and correct types

[jdufresne]

Follow upstream commit:
DefinitelyTyped/DefinitelyTyped@394c175

Can remove the ts-ignore when importing specificity.

The specificity function return a 3-tuple, so update the type alias in lib/types.d.ts.

[SethFalco]

Thanks for the fix! I was looking into making a similar pull request a few weeks ago, but shelved it as DefinitelyTyped doesn't accept types for internal APIs, which the specificity function isn't documented. 🤔

We do not support exposing undocumented internal implementation details of libraries in .d.ts files.

— https://definitelytyped.github.io/guides/contributing.html

Might be that I was wrong and it's meant to be public, but in any case, happy to run with this now that it's defined in the types. 🎉

Btw, I rebased your changes to make csso and ^@types/csso both anchor to minor version.