feat(eslint-plugin): [no-unnecessary-boolean-literal-compare] add nullish check option for fixer

[zachkirsch]

As a followup to #1983, I figured that probably the most clear solution to a nullable-boolean comparison is an actual null check.

Example: myNullableBoolean === true. The current fixer will change it to myNullableBoolean, which is a bit simpler but doesn't address the ambiguity. IMO, the most clear way to do this is myNullableBoolean != null && myNullableBoolean. This is similar in motivation to strict-boolean-expressions.

This proposal adds a fixWithExplicitNullishCheck option to the rule to modify the behavior of the fixer. You can see what this option does here: https://github.com/zachkirsch/typescript-eslint/blob/zk/explicit-nullable-boolean-cmp/packages/eslint-plugin/docs/rules/no-unnecessary-boolean-literal-compare.md#nullable-boolean-expressions

[typescript-eslint]

Thanks for the PR, @zachkirsch!

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 #2252 into master will decrease coverage by 0.02%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master    #2252      +/-   ##
==========================================
- Coverage   93.44%   93.42%   -0.03%     
==========================================
  Files         174      173       -1     
  Lines        7954     7804     -150     
  Branches     2282     2291       +9     
==========================================
- Hits         7433     7291     -142     
+ Misses        246      241       -5     
+ Partials      275      272       -3     

Flag	Coverage Δ
#unittest	93.42% <93.33%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
...rc/rules/no-unnecessary-boolean-literal-compare.ts	97.82% <93.33%> (+8.21%)	⬆️
...-internal/src/rules/no-typescript-estree-import.ts	87.50% <0.00%> (-12.50%)	⬇️
packages/parser/src/scope/scopes.ts	100.00% <0.00%> (ø)
packages/parser/src/scope/scope-manager.ts	100.00% <0.00%> (ø)
packages/typescript-estree/src/simple-traverse.ts	95.00% <0.00%> (ø)
packages/typescript-estree/src/ts-estree/index.ts	100.00% <0.00%> (ø)
...pt-estree/src/ts-estree/estree-to-ts-node-types.ts	100.00% <0.00%> (ø)
packages/typescript-estree/src/visitor-keys.ts
.../typescript-estree/src/ts-estree/ast-node-types.ts
packages/experimental-utils/src/ts-estree.ts
... and 6 more