Extract shared condition parsing into parse-condition.ts (#288)

## Summary

Prep refactor split out of #253, so that PR stays focused on the
`if()`-parsing feature itself.

Pulls media-feature (incl. range syntax), supports-condition (incl.
compound `and`/`or`/`not`), and `style()`/`selector()`/`font-tech()`
function-condition parsing out of `AtRulePreludeParser` into a new
`ConditionParser` class (`src/parse-condition.ts`), so this logic has a
single implementation instead of being duplicated by an upcoming change
to `ValueNodeParser` (needed so `if()`'s
`media()`/`supports()`/`style()` condition functions produce real
`MediaFeature`/`FeatureRange`/`SupportsQuery`/`SupportsDeclaration`
nodes instead of ad-hoc ones).

## Design notes

- `ConditionParser` takes an already-constructed `ValueNodeParser`
rather than importing and instantiating its own, since
`parse-atrule-prelude.ts` already imports `value-node-parser.ts` — a
runtime import the other way would be circular. The constructor
parameter is typed via `import type`, which is erased at build time and
creates no runtime dependency in either direction.
- `ConditionParser` owns its own `Lexer`, separate from whichever class
composes it — matching this codebase's existing sub-parser convention
(e.g. how `AtRulePreludeParser` already composes `ValueNodeParser`).
Where `AtRulePreludeParser` calls into it mid-scan on its own lexer, it
now explicitly reseeks to `ConditionParser.end_position` afterward,
since the delegated work no longer advances `AtRulePreludeParser`'s
lexer as a side effect the way an in-class method call would have.

## Test plan

- [x] Pure refactor: no behavior change intended for
`@media`/`@supports`/`@container`/`@import` parsing.
- [x] Full existing test suite passes unchanged (1391 tests, no
assertion changes).
- [x] `tsc --noEmit`, `oxlint`, `oxfmt --check` all clean.
- [x] `pnpm run build` succeeds with no circular-import issues;
`publint` clean.

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_Generated by [Claude
Code](https://claude.ai/code/session_015D2xHEuZHwna87DA8VeYjT)_

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Co-authored-by: Claude <noreply@anthropic.com>

Author: bartveneman