Add GDScript warning for standalone expression

This makes the error message clearer as it might be used to call
functions with side effects.

modules/gdscript/gdscript.h

@@ -297,6 +297,7 @@ struct GDScriptWarning {
 		UNSAFE_CAST, // Cast used in an unknown type
 		UNSAFE_CALL_ARGUMENT, // Function call argument is of a supertype of the require argument
 		DEPRECATED_KEYWORD, // The keyword is deprecated and should be replaced
+		STANDALONE_TERNARY, // Return value of ternary expression is discarded
 		WARNING_MAX,
 	} code;
 	Vector<String> symbols;