chore: replace superspawn & child_process with execa

[raphinesse]

Motivation and Context

apache/cordova#16

Includes commit from #860 (will rebase ASAP). Depends on #832 and #861.

Description

Replace superspawn & child_process usage with execa.

Testing

• npm t

Checklist

• I've run the tests to see all new and existing tests pass

[erisu]

Are these changes expected to remain here or should be moved to the Dropping Q PR? These changes kinda blend together and could be done in either PR I am guessing.

[erisu]

As #860 has been merged, I went ahead and rebased the branch with latest master.

[codecov-io]

Codecov Report

Merging #862 into master will decrease coverage by 0.09%.
The diff coverage is 67.44%.

@@            Coverage Diff            @@
##           master     #862     +/-   ##
=========================================
- Coverage   66.46%   66.37%   -0.1%     
=========================================
  Files          19       19             
  Lines        1855     1847      -8     
=========================================
- Hits         1233     1226      -7     
+ Misses        622      621      -1

Impacted Files	Coverage Δ
bin/templates/cordova/lib/device.js	100% <100%> (ø)	⬆️
bin/templates/cordova/lib/android_sdk.js	100% <100%> (ø)	⬆️
...n/templates/cordova/lib/builders/ProjectBuilder.js	67.42% <100%> (-0.37%)	⬇️
bin/templates/cordova/lib/Adb.js	100% <100%> (ø)	⬆️
bin/templates/cordova/lib/check_reqs.js	49.49% <16.66%> (-0.01%)	⬇️
bin/templates/cordova/lib/build.js	30.55% <33.33%> (ø)	⬆️
bin/templates/cordova/lib/emulator.js	89.45% <58.82%> (-0.18%)	⬇️

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