feat: Add new slots for customizing course outline section and sequence completion icons

[xitij2000]

Introduces CourseOutlineSidebarSectionCompletionIconSlot and CourseOutlineSidebarSequenceCompletionIconSlot to enable customization, modification, or replacement of completion icons in the course outline. Updated components to integrate with these new slots.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.46%. Comparing base (6f1dfbb) to head (803dec5).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1933      +/-   ##
==========================================
+ Coverage   91.30%   91.46%   +0.15%     
==========================================
  Files         346      351       +5     
  Lines        5776     5820      +44     
  Branches     1350     1393      +43     
==========================================
+ Hits         5274     5323      +49     
+ Misses        483      478       -5     
  Partials       19       19              

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[brian-smith-tcril]

Overall this is looking good! I left a couple comments with questions and notes. Looking forward to seeing this come together!

[brian-smith-tcril]

@xitij2000 it looks like CI is failing on lint errors

[xitij2000]

@brian-smith-tcril Just fixed.

[brian-smith-tcril]

Thanks for working through this with me!