feat(eslint-plugin): add no-extra-non-null-assertion

[a-tarasyuk]

Fixes #1170

[typescript-eslint]

Thanks for the PR, @a-tarasyuk!

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 #1183 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1183      +/-   ##
==========================================
- Coverage   94.17%   94.06%   -0.11%     
==========================================
  Files         124      124              
  Lines        5322     5291      -31     
  Branches     1492     1470      -22     
==========================================
- Hits         5012     4977      -35     
- Misses        177      179       +2     
- Partials      133      135       +2

Impacted Files	Coverage Δ
...nt-plugin/src/rules/no-extra-non-null-assertion.ts	100% <100%> (ø)
packages/eslint-plugin/src/rules/index.ts	100% <100%> (ø)	⬆️
packages/eslint-plugin/src/rules/camelcase.ts	82.14% <0%> (-8.77%)	⬇️
packages/eslint-plugin/src/rules/require-await.ts	92.68% <0%> (-7.32%)	⬇️
...ges/typescript-estree/src/create-program/shared.ts	77.77% <0%> (-2.23%)	⬇️
...-plugin/src/rules/no-unnecessary-type-assertion.ts	88.15% <0%> (-0.45%)	⬇️
packages/eslint-plugin/src/rules/indent.ts	92.3% <0%> (-0.38%)	⬇️
...-plugin/src/rules/no-unnecessary-type-arguments.ts	91.66% <0%> (-0.34%)	⬇️
packages/eslint-plugin/src/rules/no-type-alias.ts	94.82% <0%> (-0.26%)	⬇️
... and 8 more

[bradzacher]

nice and simple. LGTM!