feat(eslint-plugin): [no-extran-class] add allowWithDecorator opt

[a-tarasyuk]

Fixes #812

[typescript-eslint]

Thanks for the PR, @a-tarasyuk!

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 #886 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #886      +/-   ##
==========================================
+ Coverage   94.23%   94.23%   +<.01%     
==========================================
  Files         126      126              
  Lines        5376     5379       +3     
  Branches     1509     1510       +1     
==========================================
+ Hits         5066     5069       +3     
  Misses        177      177              
  Partials      133      133

Impacted Files	Coverage Δ
...ges/eslint-plugin/src/rules/no-extraneous-class.ts	100% <100%> (ø)	⬆️

[bradzacher]

LGTM - thanks for this.

one minor change, just for sanity checking.
Could you please add a test which is the inverse of the one you added.
I.e. invalid test - class with a decorator, allowWithDecorator: false

[bradzacher]

LGTM - thanks for this