Peritext: rendering surface API

[streamich]

Specify and implement an interface that each rendering surface has to provide:

• [x] Find position in prev/next line from the current cursor focus.
• [x] Ability to find soft line end/start.
• [x] Ability to navigate using ArrayUp and ArrowDown key presses.
• [x] Ability to focus/blur the editor.
  • [ ] Use it in DOM controller (blur).