fix typo in iter_2d.

replace two "for in array" loops with "for of array" loops. the former gives indexes
as strings, whereas the latter gives the actual array items.

js/utils/range.mjs

@@ -19,8 +19,8 @@ export function range (start, end) {
 }
 
 export function iter_2d (range_1d, callback) {
-	for (const y in range_1d) {
-		for (const x in range_1d) {
+	for (const y of range_1d) {
+		for (const x of range_1d) {
 			callback(x, y);
 		}
 	}