[Canvas] Refactor shortcut types to better describe specific actions

[poffdeluxe]

We should refactor our shortcut types to A) make it easier to add new shortcuts and B) make it easier to type specific shortcut actions.

For example, with something like:

  _keyHandler = (action: string) => {
    if (action === 'EDITING') {
      this.props.toggleWriteable();
    }
  };

there's not a good way to type that action besides string.

A refactor path could be to create an enum with all the actions. We could use this enum while building and typing the keymap or adding new shortcuts. This enum could also make it easier to type action types like EDITING or FULLSCREEN

[elasticmachine]

Pinging @elastic/kibana-canvas

[ThomThomson]

Closing this for the time being because we are currently unable to prioritize Canvas refactoring efforts. We can re-open this down the line if priorities change.