[WIP] missiondef: replace hardcoded mission starts with mapgen_update JSON

[mlangsdorf]

Summary

SUMMARY: Infrastructure "missiondef: replace hardcoded mission starts with mapgen_update JSON"

Purpose of change

Use the existing parsers for mapgen and NPC dialogue to move most of the hardcoded mission starts into JSON.

Describe the solution

Mission start dialogue is fully in JSON, but most missions require hardcoded C++ support to actual function.

95% of mission starts don't actually need hardcoded C++ support. They do things that can already be defined in JSON: give the player items, set traits or effects on NPCs, and adjust maps. There just isn't any support for doing that in the mission def JSON, even though other JSON does support it.

Use the existing dialogue and mapgen JSON parsers to parse new JSON in missiondef.json and remove the hardcode functions.

Describe alternatives you've considered

Still working through that.

Additional context

I really want a way to partially update an existing overmap tile for my work on faction camps, but this is also a generally useful feature that modders have requested.

Progress:

• Begin moving the existing hardcoded functions into JSON
• Finish moving the existing hardcoded functions into JSON
• Adapt the mapgen JSON parser for use in missiondef
• Adapt the dialogue effects JSON parser for use in missiondef
• Replace specific missionstart code with generic functions driven by JSON.
• Add test cases.
• Debug.
• Clean up for submission.

Comments and suggestions on better way to organize this are appreciated.

[mlangsdorf]

Currently stuck on an ordering issue: missiondef update_mapgen code needs to the npc_templates finalized before it does anything, even though normal mapgen doesn't appear to require that.

Deferred loading is a pain up my tail.