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