Add farmable XP requirement type.

[Ellendar]

It is currently possible to spawn in a position where a spell is required in order to proceed, but there is no farmable source of XP available. This most commonly happens when an early saria on mountainous / islands is required, but there is only a single terrain type that shuffled into road.

Minimally, this just requires adding a requirement type for farmable XP, and then updating the requireables check to traverse the map until it finds a terrain type that isn't shuffled to road, or a palace.

Likely this increases the need to refactor the encounter shuffle, so instead of directly shuffling the data on the ROM, it shuffles some representation of what the data means, and then writes that to the ROM.

This is extremely rare (only seen it once since 3.X), so probably low priority.