Major: Switch to Typora-style editing and discard separate preview/edit modes

### What
In place live preview. In a nutshell as you type markdown it previews the text behind your cursor eliminating the need for a separate preview space. See [https://github.com/marktext/marktext](marktext)
Or [https://typora.io/](typora) to see what I'm talking about.

### Why
Faster mental feedback and an overall better experience.

### How
All the logic is basically in the codebase but needs to be consolidated with handling for tons of edge cases

### Blocker
Needs product refinement and approval from the simplenote core team. I'm willing to do this.

The issue names no files, tests, or entry points. Begin by reviewing the existing editing and preview logic, then seek product refinement and approval from the Simplenote core team before planning the consolidation. Done would mean Typora-style live preview while typing and no separate preview/edit modes, including the stated edge cases.

Written by the indexing model from the issue text.