Core editing experience

[mattmassicotte]

Incorporating multi-cursor support (via IBeam) turned out to be ... more complex than I expected.

• text layout-based measurements for cursor movement was very hard with TextKit 2
• typing completions had to be totally refactored
• undo was a big challenge
• AppKit's cursor system really wasn't up to it

However, I now feel like I have a handle on most of these things. Going to use this issue to track the remaining work.

[mattmassicotte]

There is a crash when adding a newline to an empty document.

[mattmassicotte]

Addressed with TextStory 0.9.2.

[mattmassicotte]

Next think I want to look at is whitespace-aware cursor movement. A cmd-left arrow should end up at the start of a line's leading whitespace, for example.