feat(Reactor.Dsl.Flunk): Add a special step type which always fails. #361

Job	Run time
ash-ci / mix deps.get	6m 50s
ash-ci / MIX_ENV=test mix.compile	3m 19s
ash-ci / MIX_ENV=dev mix.compile	3m 21s
ash-ci / audit	3m 30s
ash-ci / mix dialyzer	4m 34s
ash-ci / mix credo --strict	41s
ash-ci / mix ash.codegen --check	0s
ash-ci / mix format --check-formatted	32s
ash-ci / mix sobelow	23s
ash-ci / mix spark.cheat_sheets --check	31s
ash-ci / mix spark.formatter --check	32s
ash-ci / mix test	50s
ash-ci / mix deps.unlock --check-unused	21s
ash-ci / mix docs	26s
ash-ci / Deploy docs to GitHub pages.	0s
ash-ci / Release	0s
	25m 50s

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