Add @mocha/types to improve IDE intellisense in test files

[acelaya]

This PR adds the @types/mocha package as a dev dependency, to improve IDE instellisense/autocompletion around mocha global symbols (describe, it, beforeEach, etc).

We don't type-check our test files, so this won't help actually catching incorrect usage of those symbols, it's just an affordance for editors.

This is how it looks in WebStorm without the package installed:

mocha-broken-2024-12-19_15.39.11.mp4

And this is with the package:

mocha-autocompletion-2024-12-19_15.38.14.mp4

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (4b7f380) to head (8f914db).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6729   +/-   ##
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  Coverage   99.42%   99.42%           
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  Files         270      270           
  Lines       10194    10194           
  Branches     2433     2433           
=======================================
  Hits        10135    10135           
  Misses         59       59           

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[acelaya]

We discussed this here https://hypothes-is.slack.com/archives/C1M8NH76X/p1734618358630599, so I'll go ahead and merge it.