fix: remove value from atrule nodes, fix MediaType/MediaFeature accessors

[bartveneman]

atrule.value: 'screen' -> undefined (use atrule.prelude instead)
mediaType.value: null -> 'screen'
mediaFeature.name: 'min-width' -> undefined (use mediaFeature.property instead)

• Stop setting value offsets on AT_RULE nodes in parse.ts; the prelude is now fully represented by child nodes (AT_RULE_PRELUDE or RAW)
• atrule.value returns undefined; atrule.prelude is the correct API
• mediaType.value returns the node text (e.g. "screen", "print")
• mediaFeature.name returns undefined; use mediaFeature.property instead
• has_prelude for AT_RULE now checks first_child type instead of value length

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.99%. Comparing base (d92b016) to head (5842de6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #171   +/-   ##
=======================================
  Coverage   94.99%   94.99%           
=======================================
  Files          16       16           
  Lines        2876     2877    +1     
  Branches      804      807    +3     
=======================================
+ Hits         2732     2733    +1     
  Misses        144      144           

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[codecov-commenter]

Bundle Report

Changes will decrease total bundle size by 62 bytes (-0.04%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
@projectwallace/css-parser-esm	159.03kB	-62 bytes (-0.04%) ⬇️

Affected Assets, Files, and Routes:

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

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
css-node-Uj4oBgaw.js (New)	20.82kB	20.82kB	100.0% 🚀
parse.js	-155 bytes	10.04kB	-1.52%
css-node-2ejJUrIw.js (Deleted)	-20.73kB	0 bytes	-100.0% 🗑️

Files in css-node-Uj4oBgaw.js:

• ./src/css-node.ts → Total Size: 12.46kB

Files in parse.js:

• ./src/parse.ts → Total Size: 9.64kB