added support exec_battle.lua that works in battles and gets executed with F9

credits to ShadowCX for getting it to work

Fixed globals from _lib/* like random_army_manager and invasion_manager missing in exec scripts.

Added an additional file you can run: exec2.lua with F10.

Also:
If the mod doesn't find exec.lua inside the Total War WARHAMMER II folder it will also check Total War WARHAMMER II/exec/exec.lua.
So basically you can put exec.lua and exec2.lua inside an intermediate exec folder so things are better structured.
But you don't have to.

If you're using vscode I suggest you add the exec folder to a modding workspace.