added missing virtual to ScriptExtension

_has_property_default_value is a required virtual
for ScriptExtension but it was not bound, and could
therefore not be implemented

this made it impossible to implement a ScriptExtension
that runs

core/object/script_language_extension.cpp

@@ -62,6 +62,7 @@ void ScriptExtension::_bind_methods() {
 	GDVIRTUAL_BIND(_has_script_signal, "signal");
 	GDVIRTUAL_BIND(_get_script_signal_list);
 
+	GDVIRTUAL_BIND(_has_property_default_value, "property");
 	GDVIRTUAL_BIND(_get_property_default_value, "property");
 
 	GDVIRTUAL_BIND(_update_exports);

doc/classes/ScriptExtension.xml

@@ -96,6 +96,12 @@
 			<description>
 			</description>
 		</method>
+		<method name="_has_property_default_value" qualifiers="virtual const">
+			<return type="bool" />
+			<param index="0" name="property" type="StringName" />
+			<description>
+			</description>
+		</method>
 		<method name="_has_script_signal" qualifiers="virtual const">
 			<return type="bool" />
 			<param index="0" name="signal" type="StringName" />