Add support for pseudo class `:is`

[tornqvist]

Adds support for the :is selector (aka. :matches, :any).

I also took the opportunity to add .gitignore and .npmrc files to exclude node_modules and package-lock without being dependent on local config.

[leeoniya]

thanks :)

of course the main use case for this selector is with a list of sub-selectors > 1 (which would be a good thing to add in future alongside lists in :not()).

https://developer.mozilla.org/en-US/docs/Web/CSS/:is

[tornqvist]

Agreed, this is mainly a quick fix to prevent crashing on unrecognized pseudo classes.