macos: add missing deleteForward: action

on mac the `Delete` key is really `Backspace` and should generate the
`BS` sequence.

`Fn-Delete` is equivalent to the `Delete` key on other keyboards and
should generate the `DEL` sequence.

BS maps to deleteBackward
DEL maps to deleteForward

heads up to @fanzeyi: this partially reverses 8c26b77057

window/src/os/macos/window.rs

@@ -621,7 +621,8 @@ impl WindowView {
     extern "C" fn do_command_by_selector(this: &mut Object, _sel: Sel, a_selector: Sel) {
         let selector = format!("{:?}", a_selector);
         let key = match selector.as_ref() {
-            "deleteBackward:" => KeyCode::Char('\x7f'),
+            "deleteBackward:" => KeyCode::Char('\x08'),
+            "deleteForward:" => KeyCode::Char('\x7f'),
             "cancelOperation:" => KeyCode::Char('\x1b'),
             "insertNewline:" => KeyCode::Char('\r'),
             "insertTab:" => KeyCode::Char('\t'),