[selectors][css-nesting] Move nest-containing and nest-prefixed selector definitions to Selectors

Nest-prefixed: https://drafts.csswg.org/css-nesting/#nest-prefixed
Nest-containing: https://drafts.csswg.org/css-nesting/#nest-containing

These types of relative selectors are useful beyond nesting, as the ampersand can become a universal CSS this equivalent. E.g. I can imagine a qSA-like JS API that allows us to do things like input.find("& + label") or even compositions like find("&.foo", "& > .bar", ".baz"), or even HTML attributes that allow us to refer to elements relative to the current element (how useful that would have been with label[for]!)

Sure, there is also :scope, but there’s a reason we didn't base nesting off that.

Start by reading the CSS Nesting definitions for nest-prefixed and nest-containing selectors, then compare where those definitions belong in the Selectors draft. Move the definitions so they are specified by Selectors rather than only by CSS Nesting, preserving the rationale and behavior described in the issue. Done means both relative selector types are defined in the appropriate Selectors section and remain usable for nesting.