chore: bump to typescript@3.8

[SimenB]

Summary

https://devblogs.microsoft.com/typescript/announcing-typescript-3-8/

We won't be able to use much of the new syntax (typescript-eslint/typescript-eslint#1465) but the stricter checks etc are 👍

Test plan

Green CI

[codecov-io]

Codecov Report

Merging #9607 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #9607   +/-   ##
=======================================
  Coverage   65.09%   65.09%           
=======================================
  Files         287      287           
  Lines       12145    12145           
  Branches     3009     3007    -2     
=======================================
  Hits         7906     7906           
  Misses       3604     3604           
  Partials      635      635

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[github-actions]

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