Applet Request: add terminal emulator tab (singulari_tty?)

[mathkimchi]

I assume I just need to run a shell command (from env's $SHELL or something), display its output, and relay user input.

For display, its mostly just outputting to string, but with ansi escape codes supported.

This rust terminal overview by a guy who I've seen a lot in the rust space and github goes over the most basic required features.
This and other sources are telling me to use nix::pty::forkpty but I really don't see why a simple Command won't do.

After just a bit more research, I guess the terminal emulator and shell talk to each other not with Stdio but with a pseudo-terminal, which is why we can't simply use Command's stdin and stdout.

[mathkimchi]

From a past DEVLOG entry:

singularity/DEVLOG.md

Lines 2370 to 2379 in 909cc60

	I am not completely sold on the idea of making a window manager yet, for a few reasons.
	The first is the inter-app stuf.
	Secondly, they said making a text editor and terminal from scratch is impossible for just one person.
	But, I think I can embed some other open source terminal for singularity at the worse case and run vim on it
	(but at that point, I could have just made a terminal session manager).
	I will do research on that though:
	[this reddit thread](https://www.reddit.com/r/rust/comments/1d47bl1/suggestions_on_a_gui_framework_for_embedding_a/)
	says I can embed alacritty.
	It suggests using [alacritty embeds](https://docs.rs/alacritty_terminal/latest/alacritty_terminal/#reexports).
	It gives [this example](https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/src/ui/bin/terminal/).

[mathkimchi]

It seems like alacritty intentionally separated itself to a library to make this possible:
alacritty/alacritty#450