fix(eslint-plugin): [require-await] improve performance

[bradzacher]

Figured that we didn't need to:

• check return statements in a function that isn't async.
• check return statement if the function already has an as await.
• check body-less arrow function value if it's an await expression.

This will saves many unneeded type checks across a codebase.

This has the unfortunate maintenance downside of bringing the entire codebase in, but the old extension did something unstable anyways, so I don't think it matters.

[typescript-eslint]

Thanks for the PR, @bradzacher!

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!

---

🙏 Please, if you or your company is finding typescript-eslint valuable, help us sustain the project by sponsoring it transparently on https://opencollective.com/typescript-eslint. As a thank you, your profile/company logo will be added to our main README which receives thousands of unique visitors per day.

[codecov]

Codecov Report

Merging #1536 into master will decrease coverage by 0.13%.
The diff coverage is 97.22%.

@@            Coverage Diff             @@
##           master    #1536      +/-   ##
==========================================
- Coverage   95.49%   95.36%   -0.14%     
==========================================
  Files         150      151       +1     
  Lines        6639     6711      +72     
  Branches     1880     1899      +19     
==========================================
+ Hits         6340     6400      +60     
- Misses        112      118       +6     
- Partials      187      193       +6

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/require-await.ts	97.82% <97.22%> (-2.18%)	⬇️
packages/eslint-plugin/src/util/types.ts	80.95% <0%> (-1.53%)	⬇️
packages/eslint-plugin/src/rules/index.ts	100% <0%> (ø)	⬆️
...rc/rules/no-unnecessary-boolean-literal-compare.ts	77.14% <0%> (ø)