Continuous integration

`prepublishOnly` tests before publishing to npm #14

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Run time

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Job	Run time
Test (18.20.x, macos-latest)	1m 12s
Test (18.20.x, ubuntu-latest)	50s
Test (18.20.x, windows-latest)	1m 32s
Test (20.17.x, macos-latest)	1m 8s
Test (20.17.x, ubuntu-latest)	47s
Test (20.17.x, windows-latest)	1m 47s
	7m 16s