feat: improve unknown at-rule parsing with RAW prelude and conditiona…

[bartveneman]

…l block

Previously, unknown at-rules had two problems:

1. Their prelude was wrapped in AT_RULE_PRELUDE even though the prelude parser returned no structured children — a RAW node is more accurate.
2. Their block was parsed as rules-only, so declarations like @custom { color: red } were silently dropped.

Prelude: the node type is now decided by what the prelude parser returns. If parse_prelude() produces structured nodes, AT_RULE_PRELUDE is used. If it returns [] (unknown at-rule), a RAW node is used instead. No hardcoded list required — the prelude parser already knows what it handles.

Block: the "rules-only" else branch is replaced with the conditional branch (declarations + rules + at-rules). This is safe for @Keyframes because parse_declaration() returns null for frame selectors without consuming tokens, producing an identical AST.

Examples:
@Custom prelude; → prelude is RAW (was AT_RULE_PRELUDE)
@Custom { color: red } → block contains DECLARATION (was dropped)
@Custom { .a { } } → block contains STYLE_RULE (unchanged)
@Custom { @media { } } → block contains AT_RULE (was dropped)
@Keyframes foo { from {} } → prelude still AT_RULE_PRELUDE (unchanged)

Removes CONDITIONAL_AT_RULES set and atrule_is_conditional() method as dead code after the block-parsing simplification.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.99%. Comparing base (3c310a9) to head (0e30c93).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #169      +/-   ##
==========================================
+ Coverage   94.97%   94.99%   +0.01%     
==========================================
  Files          16       16              
  Lines        2887     2876      -11     
  Branches      807      804       -3     
==========================================
- Hits         2742     2732      -10     
+ Misses        145      144       -1     

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[codecov-commenter]

Bundle Report

Changes will decrease total bundle size by 405 bytes (-0.25%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
@projectwallace/css-parser-esm	159.09kB	-405 bytes (-0.25%) ⬇️

Affected Assets, Files, and Routes:

view changes for bundle: @projectwallace/css-parser-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
parse.js	-372 bytes	10.19kB	-3.52%
parse.d.ts	-33 bytes	1.25kB	-2.57%

Files in parse.js:

• ./src/parse.ts → Total Size: 9.8kB