feat(eslint-plugin): [explicit-module-boundary-types] improve accuracy and coverage

[bradzacher]

This is a large refactor of the code so that it only checks the functions that should be checked.
Fixes #2134

Also added an option allowArgumentsExplicitlyTypedAsAny in case for some reason users want to allow explicit anys for argument types
Fixes #2119

Finally, this soft removes the shouldTrackReferences option. There's no reason you wouldn't want this set to true, because otherwise there are just a lot of cases it will miss.

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #2135 into master will increase coverage by 0.10%.
The diff coverage is 91.08%.

@@            Coverage Diff             @@
##           master    #2135      +/-   ##
==========================================
+ Coverage   93.42%   93.52%   +0.10%     
==========================================
  Files         171      171              
  Lines        7774     7788      +14     
  Branches     2222     2218       -4     
==========================================
+ Hits         7263     7284      +21     
  Misses        244      244              
+ Partials      267      260       -7     

Flag	Coverage Δ
#unittest	93.52% <91.08%> (+0.10%)	⬆️

Impacted Files	Coverage Δ
...plugin/src/rules/explicit-module-boundary-types.ts	89.92% <90.81%> (+7.16%)	⬆️
.../eslint-plugin/src/util/explicitReturnTypeUtils.ts	98.86% <100.00%> (+0.01%)	⬆️