Migrate tests from deno.land to JSR

[KnorpelSenf]

This PQ

• fixes up the old context types tests and aligns them with the composer type tests
• migrates the test deps to JSR
• stops depending on various deno.land deps to mock types and uses npm: imports instead

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.55%. Comparing base (f31feed) to head (e8e8f9f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #651   +/-   ##
=======================================
  Coverage   47.55%   47.55%           
=======================================
  Files          19       19           
  Lines        6374     6374           
  Branches      352      352           
=======================================
  Hits         3031     3031           
  Misses       3339     3339           
  Partials        4        4           

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[KnorpelSenf]

I am trying to contribute IsMutuallyAssignable to @std: denoland/std#6154

Let's see what they think of it. If they merge, we can remove our own declaration of the type, and rely on @std instead.

[Loskir]

LGTM 👍