Implement inherit from module feature for mods data
The base for inheritance of each mod should be specified in their scripts.js files as the property "inherit" of exports.BattleScripts.
If this property is omitted, it's assumed that any inheritance for the mod is relative to base data.
This intends to fix the current data for old generations, so that they inherit from Gen5, as originally.
(Per kotarou3's and Zarel's requests in #444 and #451.)
Specify basis for inheritance of mods needing it
Create scripts.js file for gen5bw in order to do so.