Add ability to listen (tap) the input

By doing something like
```
fn input_output_loop(input_output: Subject(List(String))) -> Nil {
  let output = process.receive_forever(input_output)

  echo output

  input_output_loop(input_output)
}
```

test/input/input_test.gleam

@@ -30,6 +30,7 @@ pub fn key_from_list_test() {
     TestCase([ii], key.Continue([ii])),
     TestCase([ii, "a"], key.Continue([ii, "a"])),
     TestCase([ii, "a", "b"], key.Continue([ii, "ab"])),
+    TestCase([ii, "ab", "\u{007F}"], key.Continue([ii, "a"])),
     TestCase([ii, "ab", "\r"], key.Input("ab")),
   ]