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`](https://github.com/sxyazi/yazi/pull/2461)

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`](https://github.com/helix-editor/helix/blob/b38eae1f98f63607af43e63852ff8488d8916d68/helix-term/src/commands.rs#L6163-L6228) 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.