fix: crash in node backend when printing numbers

builtin/builtin.js

@@ -1,7 +1,8 @@
 /* START builtins */
 
 function _printf(msg) {
-  process.stdout.write(msg);
+  // Message is casted to string to prevent crash
+  process.stdout.write(msg.toString());
 }
 
 /* END builtins */

examples/playground.sb

@@ -1,6 +1,6 @@
 fn main() {
 
-    let arr = ["Foo", "Bar"]
+    let arr = [1, 2, 3]
 
     for x in arr {
         _printf(x)