Explicitly support Node v12+ #270

Job	Run time
Node 12.x on ubuntu-latest	9s
Browser Tests (ubuntu-latest)	10s
Node 14.x on ubuntu-latest	10s
Browser Tests (windows-latest)	16s
Node 16 on ubuntu-latest	14s
Node lts/* on ubuntu-latest	7s
Node current on ubuntu-latest	12s
Node 12.x on macos-latest	14s
Node 14.x on macos-latest	11s
Node 16 on macos-latest	12s
Node lts/* on macos-latest	13s
Node current on macos-latest	14s
Node 12.x on windows-latest	9s
Node 14.x on windows-latest	7s
Node 16 on windows-latest	11s
Node lts/* on windows-latest	10s
Node current on windows-latest	11s
Release	0s
Code Coverage	0s
	3m 10s

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