fix(experimental-utils): getParserServices takes a readonly context

[danielnixon]

Fixes #2232

[typescript-eslint]

Thanks for the PR, @danielnixon!

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 #2235 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2235   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files         174      174           
  Lines        7954     7954           
  Branches     2282     2282           
=======================================
  Hits         7433     7433           
  Misses        246      246           
  Partials      275      275           

Flag	Coverage Δ
#unittest	93.44% <ø> (ø)

Impacted Files	Coverage Δ
...mental-utils/src/eslint-utils/getParserServices.ts	18.18% <ø> (ø)

[bradzacher]

thanks!