Add Json.decode.mapN simplifications

[lue-bird]

Completes the Json.Decode simplifications listed in #2 and more.

-- the following simplifications for map3 work for all Json.Decode.mapN
Json.Decode.map3 f (Json.Decode.succeed a) (Json.Decode.succeed b) (Json.Decode.succeed c)
--> Json.Decode.succeed (f a b c)

Json.Decode.map3 f (Json.Decode.succeed a) (Json.Decode.fail x) thirdDecoder
--> Json.Decode.fail x

Json.Decode.map3 f firstDecoder (Json.Decode.fail x) thirdDecoder
--> Json.Decode.map2 f firstDecoder (Json.Decode.fail x)

Bonus: Make using mapN checks more concise by using argCount to infer the n

[github-actions[bot]]

The branch can be tried out by running:

elm-review --template jfmengels/elm-review-simplify/preview