feat(eslint-plugin): [prom-func-async] add automatic fix

[phaux]

I use this rule a lot so I added an automatic fix for it.

const getFoo = () => fetch("/foo").then(res => res.json())
// should fix to
const getFoo = async () => fetch("/foo").then(async res => res.json())

[typescript-eslint]

Thanks for the PR, @phaux!

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 #2845 (f568a9f) into master (84a77cc) will decrease coverage by 0.00%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #2845      +/-   ##
==========================================
- Coverage   92.75%   92.75%   -0.01%     
==========================================
  Files         309      309              
  Lines       10335    10345      +10     
  Branches     2927     2931       +4     
==========================================
+ Hits         9586     9595       +9     
  Misses        346      346              
- Partials      403      404       +1     

Flag	Coverage Δ
unittest	92.75% <91.66%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
.../eslint-plugin/src/rules/promise-function-async.ts	97.36% <91.66%> (-2.64%)	⬇️

[phaux]

Changed some more code because I found some problems: (snip)

Nevermind. This PR only adds auto fix and ignores abstract methods.

[bradzacher]

LGTM - thanks for your contribution!