Add CompatHelper

[JeffFessler]

File copied from
ttps://github.com/FluxML/Zygote.jl/blob/master/.github/workflows/CompatHelper.yml

[codecov]

Codecov Report

Merging #73 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #73   +/-   ##
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  Coverage   99.68%   99.68%           
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  Files          49       49           
  Lines        2215     2215           
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  Hits         2208     2208           
  Misses          7        7           

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[johnnychen94]

As github action now ships a default Julia, there is no need to install it again and we could remove this step.

See also https://github.com/JuliaRegistries/CompatHelper.jl#11-create-the-workflow-file-required

[JeffFessler]

Ah, I should have just used the standard one instead of copying from some other repo! Thanks!
Note to self: I also created and deployed the ssh key as described in the instructions.