feat: more customisability for course outline sidebar, course tabs

[xitij2000]

This PR does a couple of things:

• It adds slots to allow customsing the tab bar, and elements of the course outline.
• It refactors the course outline so that the outline component can be used outside of a sidebar, the aim being to pull that component out into a different slot and show it at a different place.
• It generally makes the course outline more customisable via CSS variables.
• It converts all the touched code to TypeScript

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.40%. Comparing base (7168374) to head (1b58ad3).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1908      +/-   ##
==========================================
+ Coverage   91.29%   91.40%   +0.10%     
==========================================
  Files         343      350       +7     
  Lines        5770     5855      +85     
  Branches     1388     1363      -25     
==========================================
+ Hits         5268     5352      +84     
- Misses        483      484       +1     
  Partials       19       19              

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[xitij2000]

This is only used in one place and it doesn't send this param, so it can be removed.

[xitij2000]

@brian-smith-tcril Here is the PR I mentioned during the discussion on the conference talk. There may still be changes based on how well it's working out.

[musaabhasan]

One small documentation/runtime mismatch stood out while reviewing the new plugin slot surface. It is worth fixing before the PR leaves draft because downstream plugin authors are likely to copy the slot ID directly from the README.

[musaabhasan]

This documented slot ID does not match the implementation in CourseTabsNavigationSlot/index.tsx, which registers org.openedx.frontend.learning.course_tabs_navigation.v1 with tabs plural. Any env.config.jsx copied from this README would silently target the wrong slot and fail to customize the course tab navigation. Please align the README header and example key with the implemented ID, or rename the implementation if the singular ID is intended.

[xitij2000]

Thanks! I've fixed this issue.