Add props.jmc

Define reusable, dynamic text props. These must be defined in the load
function and can be accessed globally, hence the added module.

src/props.jmc

@@ -0,0 +1,6 @@
+TextProp.nbt(
+	propertyName = "SelectedItem",
+	type = player,
+	source = "@s",
+	path = SelectedItem
+);