refactor: Convert more Taxonomy code to TypeScript (2)

[bradenmacdonald]

Description

1. Converts some files from .js or .mjs to .ts
2. Refactors some tests to use the new initializeMocks helper
3. Cleans up and improves some type definitions

This is a net deletion of 138 lines of code :)

Other information

Private ref: MNG-4479

[openedx-webhooks]

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[codecov]

Codecov Report

Attention: Patch coverage is 93.18182% with 3 lines in your changes missing coverage. Please review.

Project coverage is 92.91%. Comparing base (abe68ac) to head (3dbd8d4).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/taxonomy/index.ts	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1536      +/-   ##
==========================================
- Coverage   92.91%   92.91%   -0.01%     
==========================================
  Files        1066     1071       +5     
  Lines       20988    21038      +50     
  Branches     4538     4538              
==========================================
+ Hits        19501    19547      +46     
- Misses       1414     1418       +4     
  Partials       73       73              

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