feat: Course outline restructure with new plugin slots

[xitij2000]

Splits useCourseOutlineSidebar into useCourseOutlineData + useCourseOutlineSidebar, adds CourseOutlineSidebarHeadingSlot, CourseOutlineSidebarSectionCompletionIconSlot, and CourseOutlineSidebarSequenceCompletionIconSlot, and introduces CSS variables for theming the sidebar.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.38%. Comparing base (cfc91c3) to head (cd853d0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1920      +/-   ##
==========================================
+ Coverage   91.30%   91.38%   +0.08%     
==========================================
  Files         344      349       +5     
  Lines        5774     5842      +68     
  Branches     1351     1395      +44     
==========================================
+ Hits         5272     5339      +67     
- Misses        483      484       +1     
  Partials       19       19              

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