Expanded CI to test on Windows & OSX and Julia Versions 1, Nightly, and LTS

[TheCedarPrince]

I realized I had forgotten to add testing support for Windows and OSX machines! Also, there is some need to test compatibility with the most recent LTS release (being 1.6) of Julia. This PR should introduce this testing regime.

[codecov[bot]]

Codecov Report

Merging #35 (0145ff7) into dev (8b1707c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev      #35   +/-   ##
=======================================
  Coverage   73.68%   73.68%           
=======================================
  Files           5        5           
  Lines         171      171           
=======================================
  Hits          126      126           
  Misses         45       45           

Impacted Files	Coverage Δ
src/executors.jl	100.00% <ø> (ø)

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