[css-nesting] Trivial-ish nesting API changes.

[emilio]

The API change is needed regardless (we need a declaration parser that also parses qualified rules).

The implementation details are a bit subject to change so make it opt-in.

[emilio]

r? @tiaanl @SimonSapin

I'm pretty sure we need the API change (requiring DeclarationListParser to implement QualifiedRuleParser), so I want to land it now (unpublished) to keep working on the Gecko bits.

The main question tho, is whether we want to unify these APIs a bit, which might be possible (something effectively like a trait RuleAndDeclarationParser which perhaps switches behavior a bit based on stylesheet position). But maybe for the future, once we have a better idea of what the nesting syntax requirements end up being?

[SimonSapin]

This crate hasn’t exactly been the most stable in terms of avoiding breaking changes to public APIs, so I think another one is fine.

I’m not planning to do more maintenance work (beyond my recent PR) and I don’t have a strong opinion for this API, so I’ll leave it to y’all to decide.

[emilio]

@bors-servo r=tiaanl

[bors-servo]

📌 Commit 2546cfa has been approved by tiaanl

[bors-servo]

⌛ Testing commit 2546cfa with merge 3e1bd05...

[bors-servo]

☀️ Test successful - checks-github
Approved by: tiaanl
Pushing 3e1bd05 to master...