Sync with cmark-gfm-0.29.0.gfm.12 #577

Job	Run time
lockfile	1m 15s
build_wasm	1m 2s
build_test (nightly)	2m 5s
clippy_format (stable)	59s
build_test (beta)	2m 45s
no_features_build_test (nightly)	44s
build_test (stable)	2m 36s
no_features_build_test (beta)	52s
no_features_build_test (stable)	43s
build_spec (nightly)	1m 56s
build_spec (beta)	1m 32s
build_spec (stable)	1m 25s
	17m 54s

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