revert: add eslint-import-resolver-alias as dependency

[jsjoeio]

In #3729, I removed eslint-import-resolver-alias because I didn't think we used it but turns out we do to make importing code-server plugins in a test.

I don't fully understand exactly how it works, but it has to with aliasing a module (code-server itself as a module) in one of our tests. I tried putting it in the test/package.json but ESLint didn't like that.

[codecov]

Codecov Report

Merging #3742 (9fdf049) into main (002fa08) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3742   +/-   ##
=======================================
  Coverage   61.55%   61.55%           
=======================================
  Files          35       35           
  Lines        1813     1813           
  Branches      365      365           
=======================================
  Hits         1116     1116           
  Misses        588      588           
  Partials      109      109           

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