feat(eslint-plugin): add prefer-nullish-coalescing

[bradzacher]

The intention of this rule is two fold:

1. to help migrate a codebase to nullish coalescing
2. to help educate / remind devs whilst upskilling on nullish coalescing.

By default, I disabled the rule on conditional tests. I found in our codebase that usually having || for nullish types within a conditional test was intentional, and fixing it to a nullish coalesce actually caused a bug.

Additionally, I disabled the rule on mixed operator logical expressions (i.e. expressions with both && and ||). I think that if people have mixed operators, they are probably intending to do logical expressions.

[codecov]

Codecov Report

Merging #1069 into master will decrease coverage by <.01%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master    #1069      +/-   ##
==========================================
- Coverage   94.19%   94.18%   -0.01%     
==========================================
  Files         129      130       +1     
  Lines        5631     5679      +48     
  Branches     1582     1594      +12     
==========================================
+ Hits         5304     5349      +45     
- Misses        181      182       +1     
- Partials      146      148       +2

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/index.ts	100% <100%> (ø)	⬆️
...lint-plugin/src/rules/prefer-nullish-coalescing.ts	93.61% <93.61%> (ø)