Add kindergarten-garden exercise

[keiravillekode]

We should define a data type for the 4 plants, instead of returning strings. The test file will define Eq and Show.

We will use a Vect 4, instead of a List.

We can use Maybe, with an added test case asking for the plants grown by "Zechariah". (Idris uses types like Maybe and Either, instead of having exceptions that don't appear in a function's type signature.)