Language Runtime terminals not hidden from end users

[jmcphers]

Currently, it is possible to see the Language Runtime terminals as active terminals in the Terminal tab of Positron. They should not be visible, since the fact that we are using Terminals to host Language Runtime instances is an implementation detail.

This behavior is controlled by the hideFromUser flag of the Terminal API; unfortunately, terminals that are hidden from users appear to be forced into transient mode, which prevents them from being persisted on reload.

So we need some way for a terminal to be all of the above:

• hidden from the user
• persistent through reloads &
• NOT persistent through restarts (see Language Runtime terminals incorrectly restored after quitting #213)

It'd also be nice to have an option to make the terminals visible for development, debugging, or troubleshooting purposes, even though we will hide them from end users in the default configuration.

[petetronic]

It turns out that configuring hideFromUser as true causes a terminal to be backgrounded, i.e. means window.terminals doesn't know about them and they are instead stored in a special _backgroundedTerminalInstances member of terminalService. So they are there on reload, but we would need access to currently private members to find them.
https://github.com/rstudio/positron/blob/main/src/vs/workbench/contrib/terminal/browser/terminalService.ts#L62