[css-cascade-5] Define preshint as its own cascade layer, below all other author layers

[fantasai]

We're effectively doing this already via a specificity hack, but now that we have layers we can define it as its own layer and not worry about specificity.

[fantasai]

Fwiw, this would also solve #5836 when the layer-specific revert is used in the author’s base layer.

[frivoal]

In case anybody else is like me and needs to do a bit of digging to figure out what "preshint" is, it's referring to what's discussed there: https://html.spec.whatwg.org/#the-css-user-agent-style-sheet-and-presentational-hints

And yes, I think this is a good idea.