feat(eslint-plugin): Contextual non-null [no-unnecessary-type-assertion]

[bradzacher]

Fix handling of "used before assigned" variables:
Fixes #453

Note: this fix isn't perfect, but it's "good enough".
The type checker exposes no mechanism to detect this case (but it has code for it: https://raw.githubusercontent.com/Microsoft/TypeScript/84076a55351684296f7b3f1d2715690acbe8039f/src/compiler/checker.ts and search for Variable_0_is_used_before_being_assigned)

See: microsoft/TypeScript#31124 and microsoft/TypeScript#20221

---

Also adds handling for contextually unnecessary non-null operator based on where it's used:

declare function nonNull(s: string | null);

let s: string | null = null;
nonNull(s!); // contextually unnecessary, as function accepts the same type

Will look at adding contextually unnecessary support to the as branch later - this PR is big enough as is...

[codecov]

Codecov Report

Merging #478 into master will decrease coverage by 0.14%.
The diff coverage is 85.91%.

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
- Coverage   95.88%   95.73%   -0.15%     
==========================================
  Files          79       79              
  Lines        3571     3613      +42     
  Branches      998     1013      +15     
==========================================
+ Hits         3424     3459      +35     
- Misses         51       53       +2     
- Partials       96      101       +5

Impacted Files	Coverage Δ
...ges/eslint-plugin/src/rules/no-extraneous-class.ts	100% <100%> (ø)	⬆️
packages/eslint-plugin/src/rules/member-naming.ts	100% <100%> (ø)	⬆️
packages/eslint-plugin/src/util/types.ts	73.33% <77.77%> (+10.83%)	⬆️
...-plugin/src/rules/no-unnecessary-type-assertion.ts	91.17% <87.23%> (-6.5%)	⬇️