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 91.35%. Comparing base (6f1dfbb) to head (2ceb4fe).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1933      +/-   ##
==========================================
+ Coverage   91.30%   91.35%   +0.05%     
==========================================
  Files         346      350       +4     
  Lines        5776     5821      +45     
  Branches     1350     1393      +43     
==========================================
+ Hits         5274     5318      +44     
- Misses        483      484       +1     
  Partials       19       19              

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