feat: spawn active items on mapgen

[scarf005]

Purpose of change

• wanted to prevent header modification in feat: Add ability to spawn items as active #4266 as it triggered ~300 file rebuilds.

Describe the solution

cherry-picked solutions from #4073 and #4266:

• for individual items: inlined map::spawn_item in src/mapgen.cpp and replaced with map::spawn_an_item as it allows passing item ptr (so we can activate it first)
• for item groups: sent ACTIVATE_ON_PLACE flag to Item_modifier.custom_flags in src/item_factory.cpp and checked its presence in src/item_group.cpp.
• added postprocess_fns: vector<detached_ptr -> detached_ptr> field in Item_modifier class that lets you postprocess spawned item in Command pattern.

Describe alternatives you've considered

• send lambdas in item modifier to make sending messages less hacky. skipped for this one as this meant header modification and this would have been the only usage.
• unify item and item groups spawning by spawning single items via item groups

Testing

• spawned grenades on top of shelter, one via individual item and one via item group. both are activated.
• have to write a test to ensure it gets activated but i'm not sure which API to use.
• to test, revert last commit and load a evac shelter in fresh map.

Checklist

• Document the changes in the appropriate location in the doc/ folder.
• If documentation for this feature does not exist, please write it or at least note its lack in PR description.
• New localizable fields need to be added to the lang/bn_extract_json_strings.sh script if it does not support them yet.
• If applicable, add checks on game load that would validate the loaded data.
• If it modifies format of save files, please add migration from the old format.