GDScript: Improve usability of setter chains

- Consider PackedArrays non-shared since they are copied on C++/script
  boundaries.
- Add error messages in the analyzer when assigning to read-only
  properties.
- Add specific error message at runtime when assignment fails because
  the property is read-only.

modules/gdscript/gdscript_parser.h

@@ -122,6 +122,7 @@ public:
 		TypeSource type_source = UNDETECTED;
 
 		bool is_constant = false;
+		bool is_read_only = false;
 		bool is_meta_type = false;
 		bool is_coroutine = false; // For function calls.
 
@@ -206,6 +207,7 @@ public:
 		void operator=(const DataType &p_other) {
 			kind = p_other.kind;
 			type_source = p_other.type_source;
+			is_read_only = p_other.is_read_only;
 			is_constant = p_other.is_constant;
 			is_meta_type = p_other.is_meta_type;
 			is_coroutine = p_other.is_coroutine;