Improve support for CustomFunction transforms

[phoenix-ru]

This PR enables authoring custom transformers for PseudoElement and PseudoClass CustomFunctions.

Parsing whitespace inside custom pseudo functions is now more conservative

Previously, :foo(.bar .baz .qux) was parsed as :foo(.bar.baz.qux) which greatly limited custom transformers.

This PR bypasses aggressive whitespace removal inside TokenList::parse_into behind a flag.

To ensure that no API change is introduced, conservative parsing is only enabled for CustomFunction via new TokenList::parse_preserve_whitespace.

Selector manipulations got two more public methods

Selector::append is a useful method for adding extra components to selectors, however it is quite limited when adding Component::Combinator and makes it impossible to reliably append complex expressions like .foo > .bar #baz

• Selector::insert_raw(&mut self, index: usize, component: Component) can be used for inserting new selector components into an arbitrary location, bypassing the limitations of append;
• Selector::insert_raw_multiple(&mut self, index: usize, mut components: Vec<Component>) is an optimized version of insert_raw to avoid the extra cost of allocations and memcpy when making multiple calls.

Tests

Tests were added to ensure the correctness of a different parsing strategy. No existing tests changed

[phoenix-ru]

Based on the current Pull Request, I implemented Vue's scoped style transforms using lightningcss

https://github.com/phoenix-ru/rust-vue-compiler/blob/8c97a06289695bf6fec146431c882d1b175b263d/crates/fervid_css/src/lib.rs#L53-L147

[devongovett]

I think this commit should cover this usecase as well: 6a2adb6. That will preserve all tokens inside pseudo selector functions unmodified.