Generate include/constants/wild_encounter.h automatically

[GriffinRichards]

• Write the generated wild encounter constants to their own header file, rather than in the wild encounter data file, so that they can be used elsewhere.
• Automatically generate the constants for the total number of encounter slots, including missing group total constants.
• Get rid of the redundant _mons suffix, so e.g.land_mons.mons becomes land.mons. Downstream users should do a quick find and replace in wild_encounters.json for "_mons" -> "".

Closes #1634