feat: container queries, style() and scroll-state()

[Kristofer Baxter (kristofer-baxter)]

1 of 6. This one applies to main on its own and is ready for review.

It is the first of six commits that split #57. Each adds one kind of scope, and each stands on its own: no error recovery, and no change to how the grammar reads anything it already read. The five behind it are open as drafts, because a pull request from a fork can only be based on main, and their diffs would otherwise include this one's changes.

Adds scopes for @container: the container name, size features, logical operators, style() queries and scroll-state() queries, including grouped and boolean forms. The body carries meta.at-rule.container.body.css, and both of its braces are scoped, matching the other block at-rules.

A query in parentheses may itself be a parenthesised query, so groups nest instead of closing at the first bracket, and <general-enclosed> is parsed so a bracket inside a string is not read as one. A <style-query> is parsed recursively, so not, grouped queries and ranges are not read as a declaration list. A feature value is a component value, so a function may stand in for a keyword.

style() holds a <declaration-value>, where a ; nested in brackets or a balanced block is part of the value. Its declaration-value region therefore ends only at } or ), so style(--x: [a;b]) keeps the whole value.

The container name is a <custom-ident>, so it may begin with an escape. Only none, and, not and or are reserved, so style and scroll-state are names in their own right, and only a bracket straight after either one makes it a query function. The query is optional, so a name may run into the body, and the prelude is a comma-separated list, so a name may start at a comma. The cost of a prelude is linear in its length: doubling a 12.5 KB prelude twice roughly doubles the time at each step, and 50 KB takes under half a second, which is well past any real stylesheet.

An unknown query function is <general-enclosed> wherever a query is valid, not only inside parentheses. A ) separates the identifier that follows it, so )and( needs no whitespace. A style range may be written without spaces, value first, or chained, and its custom property name stops at the comparison.

A size container feature takes a length or a ratio, and only orientation takes a keyword, so the values @media accepts are not query values here: @container (width: fullscreen) reads fullscreen as no more than an identifier.

A malformed prelude leaks its header scope to the rest of the file, which is what @media already does. @container has no semicolon form, so @container nonsense; leaks along with the rest.