Add support for analyzing @function at-rules

[bartveneman]

Summary

This PR adds support for analyzing CSS @function at-rules (from the CSS Mixins specification) to the CSS analyzer. The implementation tracks function definitions, counts their occurrences, and calculates uniqueness metrics similar to existing at-rule analysis.

Key Changes

• Added registeredFunctions collection to track @function at-rules during CSS parsing
• Implemented parsing logic to extract function names from @function prelude text, handling the function signature syntax (e.g., --my-color(--base))
• Added function property to the atrules analysis output with metrics for total count, unique count, and uniqueness ratio
• Updated test fixtures to verify correct analysis of @function at-rules, including edge cases like:
  • Duplicate function definitions
  • Case-insensitive at-rule names (@FUNCTION)
  • Invalid at-rules with no prelude
• Updated empty input test case to include the new function field in expected output

Implementation Details

• Function names are extracted by parsing the prelude text up to the opening parenthesis, handling cases where the prelude may contain parameter lists
• The implementation follows the same pattern as existing at-rule analysis (e.g., @property)
• Uniqueness ratio is calculated as the ratio of unique function names to total function definitions

https://claude.ai/code/session_01NZGg1kjwKsnugxGXx3vtLF

[codecov-commenter]

Bundle Report

Changes will increase total bundle size by 253 bytes (0.28%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
analyzeCss-esm	90.67kB	253 bytes (0.28%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: analyzeCss-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
css-*.js	253 bytes	90.67kB	0.28%

Files in css-*.js:

• ./src/index.ts → Total Size: 30.55kB