Provide an API for clearing input

[wwwillchen]

Context: https://github.com/google/mesop/pull/242/files#r1597977597

[richard-to]

For the markdown editor, I was able to clear the input using two yields, which avoids needing to keep updating the key name. So still pretty hacky implementation wise, so I definitely think this would be a nice API to have.

def on_click_new(e: me.ClickEvent):
  state = me.state(State)
  # Need to update the initial value of the editor text area so we can
  # trigger a diff to reset the editor to empty. Need to yield this change.
  # for this to work.
  state.selected_note_content = state.notes[state.selected_note_index].content
  yield
  # Reset the initial value of the editor text area to empty since the new note
  # has no content.
  state.selected_note_content = ""
  state.notes.append(Note())
  state.selected_note_index = len(state.notes) - 1
  yield