Add max-unique-units rule to stylelint plugin

[bartveneman]

Summary

This PR introduces a new stylelint rule projectwallace/max-unique-units that enforces a maximum limit on the number of unique CSS units used in a stylesheet.

Key Changes

• New rule implementation (src/rules/max-unique-units/index.ts): Validates that the number of unique CSS units does not exceed a configured threshold
• Comprehensive test suite (src/rules/max-unique-units/index.test.ts): 8 test cases covering various scenarios including edge cases and unit handling
• Plugin registration: Added the new rule to the main plugin exports in src/index.ts
• Recommended config: Set default limit to 10 unique units in the recommended configuration

Implementation Details

• The rule parses CSS declarations and extracts dimension nodes using the @projectwallace/css-parser library
• Units are normalized to lowercase for case-insensitive comparison
• The rule correctly handles:
  • Disabled state (when config value is ≤ 0)
  • Declarations without units
  • Repeated use of the same unit (only counts unique units)
  • Percentage (%) as a valid unit
• Reports a single error at the root level when the unique unit count exceeds the limit

closes #74

[codecov-commenter]

Bundle Report

Changes will increase total bundle size by 1.7kB (3.27%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
stylelintPlugin-esm	53.8kB	1.7kB (3.27%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: stylelintPlugin-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
index.mjs	1.61kB	51.0kB	3.27%
configs/recommended.d.mts	47 bytes	1.37kB	3.56%
configs/recommended.mjs	41 bytes	1.28kB	3.3%

Files in index.mjs:

• ./src/index.ts → Total Size: 823 bytes

• ./src/rules/max-unique-units/index.ts → Total Size: 1.44kB

Files in configs/recommended.mjs:

• ./src/configs/recommended.ts → Total Size: 1.24kB

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.00%. Comparing base (b6acff3) to head (37dd7b8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   97.94%   98.00%   +0.06%     
==========================================
  Files          26       27       +1     
  Lines         778      804      +26     
  Branches      222      226       +4     
==========================================
+ Hits          762      788      +26     
  Misses         13       13              
  Partials        3        3              

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