feat(experimental-utils): expose our RuleTester extension

[bradzacher]

There are a few things we've built in (like automatically adding a filename) that others might find value in.
Plus I want to use it in the internal plugin.

[typescript-eslint]

Thanks for the PR, @bradzacher!

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 #1948 into master will decrease coverage by 0.42%.
The diff coverage is 12.50%.

@@            Coverage Diff             @@
##           master    #1948      +/-   ##
==========================================
- Coverage   94.35%   93.92%   -0.43%     
==========================================
  Files         168      169       +1     
  Lines        7664     7704      +40     
  Branches     2204     2215      +11     
==========================================
+ Hits         7231     7236       +5     
- Misses        183      218      +35     
  Partials      250      250              

Flag	Coverage Δ
#unittest	93.92% <12.50%> (-0.43%)	⬇️

Impacted Files	Coverage Δ
.../experimental-utils/src/eslint-utils/RuleTester.ts	10.25% <10.25%> (ø)
...kages/experimental-utils/src/eslint-utils/index.ts	100.00% <100.00%> (ø)