add app.is_content_loaded()

doc/en/scripting/builtins/libapp.md

@@ -69,6 +69,12 @@ app.config_packs(
 Updates the packs configuration, automatically removing unspecified ones, adding those missing in the previous configuration.
 Uses app.reconfig_packs.
 
+```lua
+app.is_content_loaded() -> bool
+```
+
+Checks if content is loaded.
+
 ```lua
 app.new_world(
     -- world name