feat(eslint-plugin): [object-curly-spacing] support MappedType

[MatthiasKunnen]

The rule did support not MappedType (var d: {[k in 'union']: any}).

This PR adds tests and support.

This fixes #3175.

[typescript-eslint]

Thanks for the PR, @MatthiasKunnen!

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 #3176 (2fb235f) into master (e3a3ea0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3176   +/-   ##
=======================================
  Coverage   92.91%   92.92%           
=======================================
  Files         316      316           
  Lines       10818    10824    +6     
  Branches     3059     3059           
=======================================
+ Hits        10052    10058    +6     
  Misses        342      342           
  Partials      424      424           

Flag	Coverage Δ
unittest	92.92% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...es/eslint-plugin/src/rules/object-curly-spacing.ts	100.00% <100.00%> (ø)

[bradzacher]

LGTM - thanks for your contribution!