Add experimental `--json` option #1731

Job	Run time
maybe-expedite	4s
test (ubuntu-latest, without-token)	6m 1s
test (ubuntu-latest, with-token-0)	8m 12s
test (ubuntu-latest, with-token-1)	11m 53s
test (macos-latest, without-token)	5m 25s
test (macos-latest, with-token-0)	8m 56s
test (macos-latest, with-token-1)	13m 13s
test (windows-latest, without-token)	11m 20s
test (windows-latest, with-token-0)	18m 10s
test (windows-latest, with-token-1)	22m 34s
all-checks	1s
	1h 45m 49s

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