feat(eslint-plugin): add rule no-unsafe-assignment

[bradzacher]

See: #791, and the other rules I've added #1643, #1644, #1647

[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 #1694 into master will decrease coverage by 0.22%.
The diff coverage is 80.31%.

@@            Coverage Diff             @@
##           master    #1694      +/-   ##
==========================================
- Coverage   94.64%   94.42%   -0.23%     
==========================================
  Files         160      161       +1     
  Lines        7263     7386     +123     
  Branches     2081     2117      +36     
==========================================
+ Hits         6874     6974     +100     
- Misses        167      180      +13     
- Partials      222      232      +10     

Flag	Coverage Δ
#unittest	94.42% <80.31%> (-0.23%)	⬇️

Impacted Files	Coverage Δ
...es/eslint-plugin/src/rules/no-unsafe-assignment.ts	80.00% <80.00%> (ø)
packages/eslint-plugin/src/util/types.ts	81.01% <83.33%> (+0.74%)	⬆️
...ckages/eslint-plugin/src/rules/no-throw-literal.ts	95.74% <0.00%> (ø)
...pt-estree/src/create-program/createWatchProgram.ts	87.16% <0.00%> (+0.13%)	⬆️

[felixfbecker]

Is there anything blocking this? Been trying out the other no-unsafe-* rules (they caught real bugs!) and been missing this case

[bradzacher]

There's nothing blocking it any more. I was going to add an option to prevent "evolving" types, but I decided against that for now (I think it deserves to live as its own rule).

With the CI passing, I'll merge this now.