Warn on leftover JSON objects on firefox #12

Job	Run time
Rustfmt (stable)	15s
Check (ubuntu-latest, stable)	34s
block-autosquash-commits	0s
cargo-deny	16s
Check (ubuntu-latest, beta)	37s
Check (windows-latest, stable)	3m 27s
Check (windows-latest, beta)	1m 19s
Check (macOS-latest, stable)	50s
Check (macOS-latest, beta)	46s
Clippy (stable)	35s
Documentation (stable)	33s
Test Suite (ubuntu-latest, stable)	55s
Clippy (beta)	36s
Documentation (beta)	35s
Test Suite (ubuntu-latest, beta)	55s
Test Suite (windows-latest, stable)	2m 3s
Test Suite (windows-latest, beta)	1m 45s
Test Suite (macOS-latest, stable)	52s
Test Suite (macOS-latest, beta)	1m 9s
	18m 2s

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