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/tests/scripts/analyzer/errors/assign_to_read_only_property.gd

@@ -0,0 +1,4 @@
+func test():
+	var tree := SceneTree.new()
+	tree.root = Window.new()
+	tree.free()

modules/gdscript/tests/scripts/analyzer/errors/assign_to_read_only_property.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Cannot assign a new value to a read-only property.

modules/gdscript/tests/scripts/analyzer/errors/assign_to_read_only_property_indirectly.gd

@@ -0,0 +1,4 @@
+func test():
+	var state := PhysicsDirectBodyState3DExtension.new()
+	state.center_of_mass.x += 1.0
+	state.free()

modules/gdscript/tests/scripts/analyzer/errors/assign_to_read_only_property_indirectly.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Cannot assign a new value to a read-only property.