Supporting bidirectional (bidi) text

[mcookly]

First of all, this is an awesome project! I really look forward to its future.

I know that properly supporting bidi text is a beast with a deep, dark, and difficult history—especially for terminals and consequently terminal-based text editors. However, the clean-start aspect of Helix is too good to pass up on implementing successful bidi text management.

I see 4 ways this could be implemented:

1. Insert text "backwards" (vim's revins)
2. Display the entire buffer backwards (vim's rightleft)
3. Display bidi text "backwards" (like most applications)
4. Reverse bidi text (actually change the buffer contents)

I have no idea what it would take to implement any of these, although it seems to me the first option would be simplest. Options 3 and 4 would require something like the Unicode Bidi Algorithm. I've implemented a prototype version of option 4 as a plugin in neovim, and it works well (although performance is probably lacking).

If this (arguably essential) feature is ever added, it would be great if cursor movement was not flipped when crossing over bidi text, like in most software.

Note: Someone has asked for this feature already (#3413).