feat(eslint-plugin): [no-non-null-assert] add suggestion fixer

[bradzacher]

Always wanted a fixer for this. A normal fixer won't work because it will cause type errors.
Seems like an awesome opportunity to create a suggestion fixer.

[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 #1260 into master will decrease coverage by 0.07%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master    #1260      +/-   ##
==========================================
- Coverage    94.1%   94.03%   -0.08%     
==========================================
  Files         130      130              
  Lines        5681     5713      +32     
  Branches     1596     1608      +12     
==========================================
+ Hits         5346     5372      +26     
- Misses        182      185       +3     
- Partials      153      156       +3

Impacted Files	Coverage Δ
...ges/experimental-utils/src/ts-eslint/RuleTester.ts	0% <ø> (ø)	⬆️
packages/eslint-plugin/src/util/astUtils.ts	92.85% <80%> (-7.15%)	⬇️
...s/eslint-plugin/src/rules/no-non-null-assertion.ts	84.37% <82.75%> (-15.63%)	⬇️