examples/node: add exists() #3258

Job	Run time
lint	32s
staticcheck	43s
build	18s
tests (macos/go-1.21-amd64)	38s
tests (ubuntu/go-1.21-386)	15s
tests (ubuntu/go-1.21-amd64)	58s
tests (windows/go-1.21-386)	2m 2s
tests (windows/go-1.21-amd64)	3m 2s
tests (macos/go-1.22-amd64)	34s
tests (ubuntu/go-1.22-386)	43s
tests (ubuntu/go-1.22-amd64)	53s
tests (windows/go-1.22-386)	1m 50s
tests (windows/go-1.22-amd64)	2m 54s
test-python-example	26s
test-gsutil-example	40s
test-node-example	10s
test-go-example	37s
test-dotnet-example	12s
test-ruby-example	12s
test-scala-example	33s
test-java-example	20s
test-cpp-example	2m 14s
test-curl-example	9s
test-curl-both-scheme-default-ports-example	5s
test-curl-both-scheme-non-default-ports-example	6s
unblock-pr	1s
	21m 7s

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