fix: guard against non-string toString in IO.print and IO.write

Add writeObject_ helper that checks if obj.toString returns a String before
writing it, falling back to "[invalid toString]" otherwise. Update print,
write, and printList_ to use the new helper instead of calling toString
directly. Include test cases for both IO.print and IO.write with a class
whose toString returns an integer.

test/io/print_bad_to_string.wren

@@ -0,0 +1,5 @@
+class BadToString {
+  toString { 3 }
+}
+
+IO.print(new BadToString) // expect: [invalid toString]

test/io/write_bad_to_string.wren

@@ -0,0 +1,6 @@
+class BadToString {
+  toString { 3 }
+}
+
+IO.write(new BadToString) // expect: [invalid toString]
+IO.print