Support HTM (headless terminal multiplexer) for remote pane/tab management

[matt-kempster]

Description of the new feature/enhancement

As mentioned in #3656, it would be great to have tmux control mode support for Windows. More abstractly, it would be especially great if there was remote pane/tab management available at all, through any terminal multiplexer (not just tmux). In fact, @MisterTea's EternalTerminal has such a terminal multiplexer called "HTM" (headless terminal multiplexer). It is being adopted by Hyper.js (see vercel/hyper#2988) and would seem to have a simpler client-side implementation than tmux control mode.

To be extra clear: I really want to be able to open a connection to a remote machine, then use Windows Terminal to split a pane in two, revealing two terminal inputs for the same remote machine - or use a new tab hotkey do to the same for a tab instead of a pane. It seems like HTM might be an easy way to go forward.

Proposed technical implementation details (optional)

I'm not sure. Maybe @MisterTea would have some ideas. But from a quick glance through the HTM source code, it would appear that one might start by parsing and interpreting its header codes.

[MisterTea]

The best way to see how to interact with HTM is to look at the Hyper.js client: https://github.com/MisterTea/hyper-htm/blob/master/index.js

At a high level, HTM lays out content like this:

1. Array of tabs
2. Each tab is a tree structure where the leaf nodes are terminals and the inner nodes are splits (e.g. a tab with a vertical split pane would have a split node with two terminal leafs)

In this way it's possible to make any hierarchy of panes in a tab. Each terminal is given a UUID so when data arrives on that UUID, it should be forwarded to that terminal.

I'm happy to help develop a plugin for windows terminal and answer any questions.

[zadjii-msft]

Sure, this seems reasonable was well. Added it to the list in #4000 (along with #3656) as possible types of extensions. Presumably they'd work as some type of enhanced ITerminalConnection object. Since we've already got a top-level list of tabs and a tree of panes underneath each tab, this seems like a natural fit.

[DHowett-MSFT]

The same infrastructure would support tmuxcc, so I hope we do get a spec out of it! 😄