Migrate from Appveyor to GitHub Actions Windows runners #70

Job	Run time
lint (ubuntu-latest)	52s
tests (windows-latest)	2m 17s
alex (ubuntu-latest)	26s
yarn-lock (ubuntu-latest)	31s
lint (windows-latest)	2m 5s
alex (windows-latest)	2m 30s
tests (ubuntu-latest)	1m 37s
shellcheck	8s
yarn-lock (windows-latest)	2m 17s
build-prod (windows-latest)	2m 48s
typecheck (ubuntu-latest)	40s
licence-check (ubuntu-latest)	26s
typecheck (windows-latest)	2m 45s
licence-check (windows-latest)	3m 3s
build-prod (ubuntu-latest)	1m 3s
	23m 28s

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