refactor(snackager): drop `npm@6` version requirement #613

Job	Run time
review (snack-babel-standalone)	1m 18s
review (snack-content)	42s
review (snack-eslint-standalone)	1m 10s
review (snack-proxies)	37s
review (snack-require-context)	53s
review (snack-runtime)	1m 24s
review (snack-sdk)	55s
review (snack-term)	44s
	7m 43s

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