fix(typescript-estree): add support for extra file extensions

[sheetalkamat]

Fixes #1813

[typescript-eslint]

Thanks for the PR, @sheetalkamat!

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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 #1833 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1833   +/-   ##
=======================================
  Coverage   94.64%   94.64%           
=======================================
  Files         160      160           
  Lines        7263     7265    +2     
  Branches     2082     2082           
=======================================
+ Hits         6874     6876    +2     
  Misses        167      167           
  Partials      222      222           

Flag	Coverage Δ
#unittest	94.64% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
...pt-estree/src/create-program/createWatchProgram.ts	87.16% <100.00%> (+0.13%)	⬆️

[bradzacher]

thanks for working on this for us!