Support pseudoterminal in slides

[orhun]

I sometimes find myself using slides like this:

```sh +exec
alacritty
```

In this new terminal window I'm either running some arbitrary commands or demonstrating something about terminals.

I was thinking if it would be possible to launch a terminal/shell directly inside presenterm, e.g. with <!-- term --> marker so that I can run commands directly from the current slide. (i.e. something like an "inline terminal")

An example of that would be the tui-term widget which allows adding pseudoterminal inside TUI applications.

What do you think? 👀