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

[zhangciwu]

Fixes #1940
Using a non-null assertions(!) and an equals test(==) together makes code looks very similar with not equals test(!==), look at below code:

a! == b; // a non-null assertions(`!`) and an equals test(`==`)
a !== b; // not equals test(`!==`)
a! === b; // a non-null assertions(`!`) and an triple equals test(`===`)

I have submit an issue on typescript repo Issue: Easy misunderstanding: "! ===", and a PR to fix it.

And I think it's good for many people to avoid this writing by composing a lint rule

Doc at packages/eslint-plugin/docs/rules/confusing-non-null-assertion-like-not-equal.md in this PR

[typescript-eslint]

Thanks for the PR, @zhangciwu!

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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[zhangciwu]

A problem: in the doc, code are altered by git hooks,
(line 36 in confusing-non-null-assertion-like-not-equal.md)

const isEqualsNum = (1 + foo.num!) == 2;
becomes
const isEqualsNum = 1 + foo.num! == 2;

Now I remove code block language describer to avoid this transformation

(Some other rule "fix" it? )

---

And lack of languege describer cause a markdown lint error……

[codecov]

Codecov Report

Merging #1941 into master will decrease coverage by 0.05%.
The diff coverage is 82.60%.

@@            Coverage Diff             @@
##           master    #1941      +/-   ##
==========================================
- Coverage   93.45%   93.40%   -0.06%     
==========================================
  Files         171      172       +1     
  Lines        7813     7844      +31     
  Branches     2227     2239      +12     
==========================================
+ Hits         7302     7327      +25     
- Misses        244      245       +1     
- Partials      267      272       +5     

Flag	Coverage Δ
#unittest	93.40% <82.60%> (-0.06%)	⬇️

Impacted Files	Coverage Δ
packages/eslint-plugin/src/configs/all.ts	100.00% <ø> (ø)
...lugin/src/rules/no-confusing-non-null-assertion.ts	82.60% <82.60%> (ø)
...int-plugin/src/rules/require-array-sort-compare.ts	88.23% <0.00%> (-11.77%)	⬇️

[bradzacher]

mostly LGTM.
a few comments.

Also one additional case that should also be handled by this

let x: null | string = null;
x! = 'a';
// looks similar to x != 'a'

// should fix to
x = 'a';

[zhangciwu]

@bradzacher Great idea for x! = 'a';, will work it out; together with rule renaming

[zhangciwu]

@bradzacher
Done modifing for reviews,

But for CI / Code style and lint (pull_request)
packages/eslint-plugin/docs/rules/no-confusing-non-null-assertion.md L36
const isEqualsNum = (1 + foo.num!) == 2;
This line just can't get through check:format, prettier think the parentheses should be deleted

[bradzacher]

logic lgtm - one change for the tests

[bradzacher]

thanks for your work!