feat: add ViChangeMode event

[bonofiglio]

Closes #499.

This PR adds the ViChangeMode event. Making it possible to change the mode using the normal event system. To achieve this, a new optional function handle_mode_specific_event was added to the EditMode trait. Vi mode implements this function and handles the event to change the mode accordingly.

This will make it very easy to add the possibility of configuring mode-change keybindings in nushell.
Here's an example of how it could look:

{
  name: exit_insert_mode_with_ctrl_c
  modifier: control
  keycode: char_c
  mode: vi_insert
  event: {
    send: vichangemode
    mode: normal
  }
}

I plan on submitting a PR to nushell if this gets merged to add the functionality there too.

The main difference with #670 is that PR makes it user-configurable instead of it being a hard-coded value.
As for #853, this might prove good enough for the time being, as I'm sure many nushell users (like myself!) would really appreciate a way of remapping the vi mode change keybindings.

[sholderbach]

Thanks! The change looks reasonable given the rest of the design.

[bonofiglio]

@sholderbach Should be all set!

[sholderbach]

Thanks, do you have time to look into adding that to the nushell reedline config logic?

Your draft config example looks OK.