Node.js: fix creation of initial tiles

Make sure we create an Array before calling concat or map. Could also use Array.from, but this matches commit edc46a0

examples/memory/main.js

@@ -7,7 +7,7 @@ import * as slint from "slint-ui";
 let ui = slint.loadFile("memory.slint");
 let window = new ui.MainWindow();
 
-let initial_tiles = window.memory_tiles;
+let initial_tiles = [...window.memory_tiles];
 let tiles = initial_tiles.concat(initial_tiles.map((tile) => Object.assign({}, tile)));
 
 for (let i = tiles.length - 1; i > 0; i--) {