Support opening inside of Helix

Yazi (A file explorer written in Rust) recently added support for using yazi right inside of Helix via :insert-output

You can use Lazygit right inside of Helix as well. This is described in the Helix wiki (https://github.com/helix-editor/helix/wiki/Recipes#git-integration)

I wanted to try out gitui. So I gave it a shot and wow its about 3x faster (benchmark: undoing changes for 1,200 files). So I want to use it now. I tried using it in Helix's :insert-output but it does not work.

:insert-output uses Helix's shell_impl_async to turn the output of the passed cmd: &str and display it in the editor

You can check Yazi's PR on how they implemented it and it may be implemented here in a similar way.

:insert-output gitui would ideally allow you to use gitui right within Helix, but at the moment it just freezes the screen.