add codeowners file

[danlipert]

Description

This PR adds the CODEOWNERS file, which automatically tags the core devs for PR reviews

Refers/Fixes

No issue

Testing

Untested but doesn't do anything except trigger the github "code owners" feature

[codecov]

Codecov Report

Merging #4752 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4752   +/-   ##
=======================================
  Coverage   30.41%   30.41%           
=======================================
  Files         216      216           
  Lines       17240    17240           
  Branches     2335     2335           
=======================================
  Hits         5244     5244           
  Misses      11788    11788           
  Partials      208      208

---

Continue to review full report at Codecov.

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[codecov]

Codecov Report

Merging #4752 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4752   +/-   ##
=======================================
  Coverage   30.41%   30.41%           
=======================================
  Files         216      216           
  Lines       17240    17240           
  Branches     2335     2335           
=======================================
  Hits         5244     5244           
  Misses      11788    11788           
  Partials      208      208

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 614842b...6550181. Read the comment docs.