feat(eslint-plugin): add extension rule `comma-dangle`

[yeonjuan]

fixes #710

[typescript-eslint]

Thanks for the PR, @yeonjuan!

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 #2416 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2416   +/-   ##
=======================================
  Coverage   92.82%   92.82%           
=======================================
  Files         290      291    +1     
  Lines        9506     9551   +45     
  Branches     2664     2676   +12     
=======================================
+ Hits         8824     8866   +42     
- Misses        323      324    +1     
- Partials      359      361    +2     

Flag	Coverage Δ
#unittest	92.82% <ø> (+<0.01%)	⬆️

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)
packages/eslint-plugin/src/rules/comma-dangle.ts	93.33% <ø> (ø)

[bradzacher]

mostly lgtm! typo in the docs

[yeonjuan]

typo in the docs

@bradzacher Oh. it's my bad. Thanks :)

[bradzacher]

https://github.com/typescript-eslint/typescript-eslint/blob/master/CONTRIBUTING.md#addressing-feedback-and-beyond

When I've got time to review larger feature PRs, I'll get to it.
I've been swamped the last few weeks with the v4 release and the fixes that were required for it.

[bradzacher]

LGTM - thanks for your contribution!