test migrating to pnpm; fix a lot typing stuff

[mipyykko]

pnpm has a lot of built in stuff that previously warranted separate packages (like npm-check and patch-package here), is faster and all that. Somehow applying it also revealed a lot of (type) bugs here and there.

Also updates quite a few deps.

[codecov]

Codecov Report

Merging #1213 (4778004) into master (29b9ede) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1213      +/-   ##
==========================================
+ Coverage   64.23%   64.24%   +0.01%     
==========================================
  Files         124      124              
  Lines        5318     5320       +2     
  Branches     1234     1234              
==========================================
+ Hits         3416     3418       +2     
  Misses       1773     1773              
  Partials      129      129              

Flag	Coverage Δ
backend	64.24% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
backend/api/routes/storedData.ts	97.67% <ø> (ø)
backend/graphql/Completion/index.ts	100.00% <ø> (ø)
backend/graphql/Course/index.ts	100.00% <ø> (ø)
backend/graphql/Exercise.ts	63.04% <ø> (ø)
backend/graphql/StudyModule/index.ts	100.00% <ø> (ø)
backend/graphql/User/index.ts	100.00% <ø> (ø)
backend/graphql/index.ts	100.00% <ø> (ø)
backend/schema.ts	82.85% <100.00%> (+1.03%)	⬆️

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