[Range] Allow styling individual thumbs using :nth-child()

[TheOneTheOnlyJJ]

This is a very simple feature request.

The current explainer says:

These pseudo-elements can be used in combination with other CSS selectors, such as :nth-child(), to provide granular control over individual ticks, labels, and segments.

I believe ::range-thumb should also allow this granular control, using the same approach.

[lukewarlow]

Something interesting here is that currently you can't use structural pseudo classes on pseudo elements. So :nth-child() wouldn't work on any pseudo elements. I'm not sure if this is a restriction we can necessarily loosen. Though perhaps with base appearance having a defined structure it's plausible.