manual bump to gts 7, and necessary code nits

[seunomonije]

Modifies the config file to the flat format described here.

Also, .eslintrc.json has been deprecated in eslint 9, being replaced by eslint.config.js. Opted to use CommonJS here because it's cleaner, matches up with the gts README example and we shouldn't need to migrate for a while.

There are some stricter lint rules introduced by the Typescript updates:

@typescript-eslint/no-unused-expressions
@typescript-eslint/no-wrapper-object-types

that I also patched up in the code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.57%. Comparing base (479c2bc) to head (11e7ff7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7826   +/-   ##
=======================================
  Coverage   99.57%   99.57%           
=======================================
  Files        1102     1102           
  Lines       98772    98772           
=======================================
  Hits        98350    98350           
  Misses        422      422           

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[pavoljuhas]

Awesome! Thank you for fixing this!