the-farm: add concept(s) / prerequisites

[jmrunkle]

Aftermath for #1634 (just fixing CI) - this exercise needs to be enabled and requires the concept it is trying to teach and prereqs for that concept.

[ErikSchierboom]

Looking at the exercise, the prereqs should at least include:

• multiple-return-values: required for the multiple return values, will be introduced in card-tricks: introduce multiple-return-values concept #1619
• numbers: for float64 and int
• conditionals-if: for if statements
• nil: used to check if there isn't an error