feat: add support for @function at-rule

[bartveneman]

Parses the CSS @function at-rule (https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/At-rules/@function).

The @function body is treated as a conditional block (like @media), allowing both declarations and nested at-rules such as @media inside the function body. The prelude is parsed to extract the function name as an IDENTIFIER node.

Usage:

  const ast = parse('@function --transparent(--color, --alpha) { result: oklch(...); }')
  const fn = ast.first_child
  fn.name                      // 'function'
  fn.value                     // '--transparent(--color, --alpha)' (raw prelude)
  fn.prelude.first_child.text  // '--transparent' (function name as IDENTIFIER)
  fn.block.children            // [DECLARATION result: ...]

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.97%. Comparing base (42d04c6) to head (9041f24).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   94.96%   94.97%   +0.01%     
==========================================
  Files          16       16              
  Lines        2877     2887      +10     
  Branches      804      807       +3     
==========================================
+ Hits         2732     2742      +10     
  Misses        145      145              

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[codecov-commenter]

Bundle Report

Changes will increase total bundle size by 423 bytes (0.27%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
@projectwallace/css-parser-esm	159.49kB	423 bytes (0.27%) ⬆️

Affected Assets, Files, and Routes:

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

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
parse-atrule-prelude.js	376 bytes	21.05kB	1.82%
parse.js	13 bytes	10.57kB	0.12%
parse-atrule-prelude.d.ts	34 bytes	1.65kB	2.1%

Files in parse-atrule-prelude.js:

• ./src/parse-atrule-prelude.ts → Total Size: 20.65kB

Files in parse.js:

• ./src/parse.ts → Total Size: 10.17kB