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.

modules/gdscript/tests/scripts/runtime/assign_to_read_only_property.gd

@@ -0,0 +1,7 @@
+func test():
+	var state = PhysicsDirectBodyState3DExtension.new()
+	assign(state)
+	state.free()
+
+func assign(state):
+	state.center_of_mass.x -= 1.0

modules/gdscript/tests/scripts/runtime/assign_to_read_only_property.out

@@ -0,0 +1,6 @@
+GDTEST_RUNTIME_ERROR
+>> SCRIPT ERROR
+>> on function: assign()
+>> runtime/assign_to_read_only_property.gd
+>> 7
+>> Cannot set value into property "center_of_mass" (on base "PhysicsDirectBodyState3DExtension") because it is read-only.

modules/gdscript/tests/scripts/runtime/assign_to_read_only_property_with_variable_index.gd

@@ -0,0 +1,8 @@
+func test():
+	var state = PhysicsDirectBodyState3DExtension.new()
+	var prop = &"center_of_mass"
+	assign(state, prop)
+	state.free()
+
+func assign(state, prop):
+	state[prop].x = 1.0

modules/gdscript/tests/scripts/runtime/assign_to_read_only_property_with_variable_index.out

@@ -0,0 +1,6 @@
+GDTEST_RUNTIME_ERROR
+>> SCRIPT ERROR
+>> on function: assign()
+>> runtime/assign_to_read_only_property_with_variable_index.gd
+>> 8
+>> Cannot set value into property "center_of_mass" (on base "PhysicsDirectBodyState3DExtension") because it is read-only.