reorganize module

_archive/rhai_engine/rhaibook/ref/getters-setters.md

@@ -0,0 +1,23 @@
+Properties
+==========
+
+Data types typically expose properties, which can be accessed in a Rust-like syntax:
+
+> _object_ `.` _property_
+>
+> _object_ `.` _property_ `=` _value_ `;`
+
+A runtime error is raised if the property does not exist for the object's data type.
+
+
+Elvis Operator
+--------------
+
+The [_Elvis operator_](https://en.wikipedia.org/wiki/Elvis_operator) can be used to short-circuit
+processing if the object itself is `()`.
+
+> `// returns () if object is ()`  
+> _object_ `?.` _property_
+>
+> `// no action if object is ()`  
+> _object_ `?.` _property_ `=` _value_ `;`