Explicit PositionIndex in EnemySpawn.json

[alborrajo]

Please describe the idea.
Right now, the position index is determined by the order in which the enemies of a same layout (stage, layer, group and subgroup) are present in the file. Instead of that it'd be better to have an optional parameter for an explicit position index, if missing, it'd behave like it did up until now.

Describe the solution you'd like
An extra, optional, parameter for each item in the enemy list for PositionIndex. Additionally, it'd be useful to interpret multiple entries for the same layout with the same PositionIndex value to mean "choose randomly one entry between any of these"

Describe alternatives you've considered
None

Additional context
None