Custom key binding support for text_editor

[hecrj]

This PR introduces custom key binding support for the text_editor widget.

You can use the new key_binding method, which accepts a closure that takes a KeyPress and optionally produces a Binding. You can obtain the default binding behavior with Binding::from_key_press.

For instance, let's say we want to produce a Submit message only when Enter is pressed without the Shift modifier:

text_editor(&self.content)
    .key_binding(|event| {
        let modifiers = event.modifiers;

        match text_editor::Binding::from_key_press(event) {
            Some(text_editor::Binding::Enter) if !modifiers.shift() => {
                Some(text_editor::Binding::Custom(Message::Submit))
            }
            binding => binding,
        }
    });

The Binding enum has a Sequence(Vec<Binding>) variant that can be used to produce combinations as well.

I believe these changes should allow users to implement custom key mappings in their editors, like Vim or Kakoune.