GDScript: add an optional message parameter to assert()

Before this patch, assert() only took the condition to assert on:

    assert(item_data)

Now, it can optionally take a string that will be printed upon failure:

    assert(item_data, item_name + " has no item data in ItemDatabase")

This makes it easier to immediately see what the issue is by being
able to write informative failure messages.

Thanks to @wiped1 for sharing their patch, upon which this is based.

Closes #17082

modules/gdscript/gdscript_parser.h

@@ -481,7 +481,12 @@ public:
 
 	struct AssertNode : public Node {
 		Node *condition;
-		AssertNode() { type = TYPE_ASSERT; }
+		Node *message;
+		AssertNode() :
+				condition(0),
+				message(0) {
+			type = TYPE_ASSERT;
+		}
 	};
 
 	struct BreakpointNode : public Node {