fix: always create an atrule prelude node if the atrule has a prelude

[bartveneman]

Changes:

1. src/parse-atrule-prelude.ts:82-84, 471-484: Added @charset prelude parser that returns a STRING node
2. src/parse.ts:367-376: Always create AT_RULE_PRELUDE wrapper when prelude parsing is enabled, even for at-rules without structured parsing
3. src/parse.test.ts:1085-1087, 1096-1099: Updated @charset and @namespace tests to expect preludes
4. src/parse.test.ts:1102-1112: Added test for @imaginary-atrule proving unknown at-rules have accessible .prelude.text

All 1079 tests pass. Now you can access .prelude.text for any at-rule with a prelude, regardless of whether it has structured parsing support.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.91%. Comparing base (2a57c89) to head (13e6fb3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
+ Coverage   94.89%   94.91%   +0.01%     
==========================================
  Files          15       15              
  Lines        2566     2575       +9     
  Branches      671      674       +3     
==========================================
+ Hits         2435     2444       +9     
  Misses        131      131              

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[codecov-commenter]

Bundle Report

Changes will increase total bundle size by 433 bytes (0.31%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
@projectwallace/css-parser-esm	139.66kB	433 bytes (0.31%) ⬆️

Affected Assets, Files, and Routes:

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

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
parse-*.js	435 bytes	22.44kB	1.98%
parse.js	-2 bytes	13.16kB	-0.02%

Files in parse-*.js:

• ./src/parse-atrule-prelude.ts → Total Size: 21.67kB

Files in parse.js:

• ./src/parse.ts → Total Size: 12.3kB