Add focus management

[vadimdemedes]

This PR adds a simple focus management mechanism to Ink. It's based on @jdeniau's work in #262 and Focus Management RFC in React.

You can check out the documentation for changes made here in https://github.com/vadimdemedes/ink/tree/focus#usefocusoptions, but the gist is that components can call useFocus() hook to notify Ink that they can be focused and when user presses Tab, Ink is going to move focus between focusable components.

import {useFocus} from 'ink';

const FocusableComponent = () => {
  const {isFocused} = useFocus();
  // `isFocused` will equal `true` after user presses Tab and this component receives focus

  return <Text>{isFocused ? 'I am focused' : 'I am not focused'}</Text>;
};

[vadimdemedes]

@jdeniau @taras Let me know what you think!

[jdeniau]

Looks good to me.

[jdeniau]

Personally, I'd move this context provider and all the following methods into a separate file because this file becomes more complicated with those 200 supplementary lines. (ie. someone that want to improve ink might be lost in those lines whereas he does not need to understand focus management)

[vadimdemedes]

Agree and this is what I wanted to do initially, but decided to leave it in App.tsx for now to reuse the data event handler on stdin stream, instead of having two separate ones. I think a better move here is to rewrite this file using hooks and extract individual functionality (focus management, exiting, raw mode) into hooks, which will clean this whole thing up.