Add `crossword` sample #401

Job	Run time
Test Flutter stable on ubuntu-latest	2m 40s
Test Flutter stable on macos-latest	3m 59s
Test Flutter stable on windows-latest	5m 8s
Test Flutter beta on ubuntu-latest	2m 38s
Test Flutter beta on macos-latest	4m 8s
Test Flutter beta on windows-latest	1m 6s
	19m 39s

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