Add experimental `--json` option #1730

Job	Run time
maybe-expedite	2s
test (ubuntu-latest, without-token)	8m 17s
test (ubuntu-latest, with-token-0)	14m 27s
test (ubuntu-latest, with-token-1)	18m 30s
test (macos-latest, without-token)	8m 15s
test (macos-latest, with-token-0)	18m 40s
test (macos-latest, with-token-1)	19m 7s
test (windows-latest, without-token)	18m 12s
test (windows-latest, with-token-0)	26m 46s
test (windows-latest, with-token-1)	29m 15s
all-checks	1s
	2h 41m 32s

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