Add unstable version of umbrella

Author: demiurgeQuantified

.gitmodules

@@ -206,3 +206,6 @@
 	path = addons/ncine/module
 	url = https://github.com/nCine/nCine-LuaCATS
 	branch = addon
+[submodule "addons/umbrella-unstable/module"]
+	path = addons/umbrella-unstable/module
+	url = https://github.com/demiurgeQuantified/UmbrellaAddon-Unstable

addons/umbrella-unstable/info.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://raw.githubusercontent.com/LuaLS/LLS-Addons/main/schemas/addon_info.schema.json",
+  "name": "Umbrella (Unstable)",
+  "description": "Definitions for the Unstable version of Project Zomboid's API",
+  "size": 4842236,
+  "hasPlugin": false
+}