Wait for newlines, then render markdown on a line by line basis. Word wrap it for the current terminal size and then spit it out line by line into the UI. Also adds tests and fixes some UI regressions.
Added:
* C-m for newline (not sure if this is actually treated differently to
Enter, but tui-textarea handles it and it doesn't hurt)
* C-d to delete one char forwards (same as Del)
* A-bksp to delete backwards one word
* A-arrows to navigate by word
This replaces tui-textarea with a custom textarea component.
Key differences:
1. wrapped lines
2. better unicode handling
3. uses the native terminal cursor
This should perhaps be spun out into its own separate crate at some
point, but for now it's convenient to have it in-tree.
