GDScript LSP: Implement signatureHelp

Enable smart resolve default to true as it is required for script symbol lookup

modules/gdscript/language_server/gdscript_text_document.h

@@ -67,6 +67,7 @@ public:
 	Variant hover(const Dictionary &p_params);
 	Array definition(const Dictionary &p_params);
 	Variant declaration(const Dictionary &p_params);
+	Variant signatureHelp(const Dictionary &p_params);
 
 	void initialize();