Add tests for pnpm and yarn (#117) #6

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Job	Run time
test (ubuntu-latest, lts/*, npm)	12m 23s
test (ubuntu-latest, lts/*, pnpm)	12m 47s
test (ubuntu-latest, lts/*, yarn)	12m 0s
test (ubuntu-latest, 18, npm)	12m 28s
test (ubuntu-latest, 18, pnpm)	12m 45s
test (ubuntu-latest, 18, yarn)	11m 15s
test (windows-latest, lts/*, npm)	14m 31s
test (windows-latest, lts/*, pnpm)	28m 10s
test (windows-latest, lts/*, yarn)	14m 23s
test (windows-latest, 18, npm)	15m 17s
test (windows-latest, 18, pnpm)	37m 31s
test (windows-latest, 18, yarn)	14m 27s
test (macos-latest, lts/*, npm)	16m 28s
test (macos-latest, lts/*, pnpm)	13m 22s
test (macos-latest, lts/*, yarn)	10m 50s
test (macos-latest, 18, npm)	15m 8s
test (macos-latest, 18, pnpm)	13m 57s
test (macos-latest, 18, yarn)	12m 41s
automerge	0s
	4h 40m 23s