Avoid invalid `::part` selector

[m-akinc]

It is not valid CSS to add a class selector to the ::part() pseudo-element, so rewriting a selector like ::part(foo):hover to ::part(foo).pseudo-hover is a problem.

::-webkit-scrollbar-thumb and ::-webkit-slider-thumb have the same limitation, and there was already logic to avoid generating modified selectors that included those. However, that logic was overly restrictive, preventing valid rewrites like div:hover::-webkit-scrollbar-thumb -> div.pseudo-hover::webkit-scrollbar-thumb.

This change includes:

• prevents generating invalid CSS with ::part() followed by a class selector
• support for selectors like elt:hover::part(foo) and elt:hover::-webkit-scrollbar-thumb
• make sure pseudo-*-all classes get added to shadow hosts, even if their shadow DOM didn't have stylesheets that got rewritten. This is necessary to support ::part()-based styling, since those styles are (usually) in the light DOM.
• make sure stale pseudo-*-all classes get cleared from shadow hosts when switching Storybook stories
• new Storybook story for ::part() support

[m-akinc]

@ghengeveld Assuming this gets merged, I have at least one other change to submit afterwards.

[ghengeveld]

@ghengeveld Assuming this gets merged, I have at least one other change to submit afterwards.

Do you want to release that other change along with this one? In that case this PR should be labeled with skip-release.

[m-akinc]

Do you want to release that other change along with this one? In that case this PR should be labeled with skip-release.

I assumed it would be easier to just release once, which is why I gave that heads-up. I don't have permissions to modify the labels on the PR, so I guess you will need to add the label (unless you want to give contributors that permission).

[github-actions[bot]]

:rocket: PR was released in v3.1.0 :rocket: