feat: add vendor-prefix support in mediafeature

[bartveneman]

• Add value getter to CSSNode for MEDIA_FEATURE nodes, returning the
  first child node or null for boolean features like (hover)
• Add MEDIA_FEATURE case to is_vendor_prefixed getter, checking the
  feature name (e.g. -ms-high-contrast → true)
• Add value: CSSNode | null to the MediaFeature type; remove WithChildren
  for consistency with Declaration, which also hides children in favour of
  a typed value accessor
• Update tests to use feature.value instead of feature.children

[codecov-commenter]

Bundle Report

Changes will increase total bundle size by 194 bytes (0.11%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
@projectwallace/css-parser-esm	184.38kB	194 bytes (0.11%) ⬆️

Affected Assets, Files, and Routes:

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

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
node-types-CORQtdPo.d.ts (New)	23.37kB	23.37kB	100.0% 🚀
css-node-2WA1GE9K.js (New)	14.14kB	14.14kB	100.0% 🚀
node-types-D3K9wUYU.d.ts (Deleted)	-23.28kB	0 bytes	-100.0% 🗑️
css-node-DSWz7wzo.js (Deleted)	-14.04kB	0 bytes	-100.0% 🗑️

Files in css-node-2WA1GE9K.js:

• ./src/css-node.ts → Total Size: 13.94kB

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.83%. Comparing base (ad61272) to head (7073dff).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #225   +/-   ##
=======================================
  Coverage   93.82%   93.83%           
=======================================
  Files          17       17           
  Lines        2947     2951    +4     
  Branches      802      805    +3     
=======================================
+ Hits         2765     2769    +4     
  Misses        182      182           

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