feat: Add slot for customizing course outline sidebar unit icons

[xitij2000]

Introduces CourseOutlineSidebarUnitIconSlot to enable customization, modification, or replacement of unit icons in the course outline sidebar.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.31%. Comparing base (cfc91c3) to head (1c07116).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1919      +/-   ##
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+ Coverage   91.30%   91.31%   +0.01%     
==========================================
  Files         344      345       +1     
  Lines        5774     5783       +9     
  Branches     1351     1385      +34     
==========================================
+ Hits         5272     5281       +9     
  Misses        483      483              
  Partials       19       19              

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[brian-smith-tcril]

This feels very narrowly scoped, and I'm thinking it might make more sense to have a slot that contains the entire unit link instead of just the icon. Then instead of replacing UnitIcon with CourseOutlineSidebarUnitIconSlot in SidebarUnit we could replace SidebarUnit with SidebarUnitSlot in SidebarSequence.

[xitij2000]

@brian-smith-tcril The reason I kept is narrow is that making it broader would also kind of freeze a larger part of the props as API. However, I understand that just the icons would be too narrow.

I'll look into changing this. I have some time off coming up next week so might only be able to fully look into and and wrap it up after I return.

[brian-smith-tcril]

It's probably worth talking through the possible options here. I'd like to find a good balance between "this does everything people could possibly want when customizing the unit links in the sidebar" and "this locks us in to way too many pluginProps."

The ideas that come to mind for me are:

• Just wrap the icon
  • Not flexible enough, what if someone wants to change the text?
• Wrap the entire thing
  • Possibly requires too many plugin props
• Wrap what's inside UnitLinkWrapper
  • The existing styling baked in to UnitLinkWrapper itself might make it hard to use
• Refactoring to split the visual aspects of SidebarUnit and UnitLinkWrapper out from the functional aspects to provide visual flexibility without exposing all the functional aspects as plugin props
  • Might be complicated, not sure what a good shape would be for that