feat(eslint-plugin): add rule non-nullable-type-assertion-style

[JoshuaKGoldberg]

Fixes #202.

[typescript-eslint]

Thanks for the PR, @JoshuaKGoldberg!

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[codecov]

Codecov Report

Merging #2624 (8112c89) into master (84a77cc) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2624      +/-   ##
==========================================
+ Coverage   92.75%   92.77%   +0.02%     
==========================================
  Files         309      310       +1     
  Lines       10335    10367      +32     
  Branches     2927     2934       +7     
==========================================
+ Hits         9586     9618      +32     
  Misses        346      346              
  Partials      403      403              

Flag	Coverage Δ
unittest	92.77% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/eslint-plugin/src/configs/all.ts	100.00% <ø> (ø)
...gin/src/rules/non-nullable-type-assertion-style.ts	100.00% <100.00%> (ø)

[bradzacher]

Code itself LGTM.

---

I wonder if this rule could also check for this case:

type T = string | null;
declare const x: T;

const y = x as NonNullable<T>;
// should be
const y = x!;

[JoshuaKGoldberg]

Adding NonNullable SGTM - I'll be mostly out of availability the next couple of weeks, but hopefully will either find time to sneak it in with then or should be able to get to later. 👍

[JoshuaKGoldberg]

Amusingly, even though I took a month and a half to get back to this, it's already supported: the logic just checks for whether the types are the same but for null and/or undefined added. So NonNullable and any of equivalents are supported already. Added test cases!

[bradzacher]

LGTM - thanks for your work!