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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.40%. Comparing base (7168374) to head (5c892a2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1908      +/-   ##
==========================================
+ Coverage   91.29%   91.40%   +0.10%     
==========================================
  Files         343      349       +6     
  Lines        5770     5851      +81     
  Branches     1388     1400      +12     
==========================================
+ Hits         5268     5348      +80     
- 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.