style: use const when possible

[vil02]

As suggested here, I stared exploring linting/static analysis tools available for TypeScript. The TypeScipt eslint seems to be reasonable. These changes are the result of running npx eslint . --fix, and from what I see it has changed let to const, which is a tiny step in a good direction.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4f6cd6c) 96.32% compared to head (323fd01) 96.32%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #230   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files          97       97           
  Lines        1797     1797           
  Branches      347      347           
=======================================
  Hits         1731     1731           
  Misses         66       66           

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