fix(experimental-utils): correct types for TS versions older than 3.8

[bradzacher]

Fixes #2216

[typescript-eslint]

Thanks for the PR, @bradzacher!

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[codecov]

Codecov Report

Merging #2217 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2217   +/-   ##
=======================================
  Coverage   93.39%   93.39%           
=======================================
  Files         173      173           
  Lines        7871     7871           
  Branches     2247     2247           
=======================================
  Hits         7351     7351           
  Misses        247      247           
  Partials      273      273           

Flag	Coverage Δ
#unittest	93.39% <ø> (ø)