Allow an extension to control a terminal tab title/description variable

[Tyriar]

Some extensions create "special" terminals, one way of differentiating them is explicitly setting a title but then that replaces the standard title. What if instead a variable was controlled by an extension which would then default beside the task/local variables as they're similar:

Context: microsoft/vscode-python#20492 (comment)

[karrtikr]

For our use case we would like to apply this to all terminals (even if it's not the one we created).

[Tyriar]

Current thinking:

• Make name non-readonly
• This is used as the ${process} variable in the terminal title/description settings

export interface Terminal {
	/**
	 * The name of the terminal, this will be used as the `${process}` variable in the terminal
	 * tab title and description settings.
	 */
	name: string;
}

We could use a ${extension} variable instead so you can still use process, this might clutter the UI too much though.

[Tyriar]

Feedback from API sync:

• Throw a runtime error if a non-owning extension tries to change the name, mention in API docs
• Improve ${process} setting description
• Finalizing is fine, provided it works

[karrtikr]

Proposal:

• Add non-readonly property "description"
• This is used as the ${extension} variable in the terminal title/description settings
• Any non-owning extension can contribute to the description. Information will show up on the right without shifting other text:
  

[Tyriar]

I'm concerned about non-owning extensions clobbering the description, leading to inconsistent behavior. For example an extension may create the terminal, set the description, and Python would overwrite it. Or the opposite, where Python sets it and some other extensions overwrites it.

After thinking about it more, it feels wrong that non-owning extensions have any say in this and we should instead attach this information in a less prominent way using the env var description as proposed in #171108. I also think allowing an extension to set text in the description is going to lead to an overly cluttered UI. So we could surface the information in the tooltip where we show shell integration's state:

Yes this is not as prominent as you might like, but I want to establish this area as how you get information about the particular terminal. We also have a WIP PR to add the process PID/executable/args there for example: #171258

[Tyriar]

The current plan is to not do this