test(nextjs): Catch type errors in next.config.ts

[Lms24]

Our NextJS e2e test used a next.config.mjs file which means when building the nextjs app, type errors in this file are not caught. This caused the bug fixed via #798 to slip through. So this PR makes the next config a typescript file which will throw a build error if a type mismatch (like adding an unsupported property to the Sentry build options) is detected.

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.55%. Comparing base (801fbe4) to head (51f4e4d).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #810      +/-   ##
==========================================
- Coverage   49.67%   49.55%   -0.12%     
==========================================
  Files          52       52              
  Lines        3503     3519      +16     
  Branches      825      825              
==========================================
+ Hits         1740     1744       +4     
+ Misses       1762     1639     -123     
- Partials        1      136     +135     

Flag	Coverage Δ
e2e-tests	80.85% <ø> (-3.77%)	⬇️
unit-tests	48.25% <ø> (-0.08%)	⬇️

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[Lms24]

hmm crap, next config TS only works with Next 15. I don't think we want to stop testing against Next 14 though, so we probably need to create a second e2e test app with Next 15. I'll try to slot this in when I have a spare cycle but can't spend too much time on this right now.