Adopt virtual process terminals in task custom execution API

[Tyriar]

Implemented in #70978

There's a proposal for how it would work there.

[alexr00]

	/**
	 * Class used to execute an extension callback as a task.
	 */
	export class CustomExecution2 {
		/**
		 * @param process The [TerminalVirtualProcess](#TerminalVirtualProcess) to be used by the task to display output.
		 * @param callback The callback that will be called when the task is started by a user.
		 */
		constructor(process: TerminalVirtualProcess, callback: (thisArg?: any) => Thenable<void>);

		/**
		 * The callback used to execute the task. Cancellation should be handled using the [TerminalVirtualProcess](#TerminalVirtualProcess).
		 * When the task is complete, onDidExit should be fired on the TerminalVirtualProcess with the exit code with '0' for success and a non-zero value for failure.
		 */
		callback: (thisArg?: any) => Thenable<void>;

		/**
		 * The [TerminalVirtualProcess](#TerminalVirtualProcess) will be attached to a terminal when the task is started.
		 * At that point it can be used to show output and accept input. process.shutdown() should be implemented to handle interruption of the task
		 */
		process: TerminalVirtualProcess;
	}

[alexr00]

Updated API:

	/**
	 * Class used to execute an extension callback as a task.
	 */
	export class CustomExecution2 {
		/**
		 * @param process The [TerminalVirtualProcess](#TerminalVirtualProcess) to be used by the task to display output.
		 * @param callback The callback that will be called when the task is started by a user.
		 */
		constructor(callback: (thisArg?: any) => Thenable<TerminalVirtualProcess>);

		/**
		 * The callback used to execute the task. Cancellation should be handled using the shutdown method of [TerminalVirtualProcess](#TerminalVirtualProcess).
		 * When the task is complete, onDidExit should be fired on the TerminalVirtualProcess with the exit code with '0' for success and a non-zero value for failure.
		 */
		callback: (thisArg?: any) => Thenable<TerminalVirtualProcess>;
	}

This also requires a start on the TerminalVirtualProcess because:

const execution =  new vscode.CustomExecution2(
						async (): Promise<vscode.TerminalVirtualProcess> => {
							return new TestTerminalProcess();
						});
...
class TestTerminalProcess implements vscode.TerminalVirtualProcess {

	private writeEmitter = new vscode.EventEmitter<string>();
	onDidWrite = this.writeEmitter.event;
	private exitEmitter = new vscode.EventEmitter<number>();
	onDidExit = this.exitEmitter.event;

	constructor() {
		// Can't start emitting things here, no one is listening. 
	}

	start() {
		this.writeEmitter.fire('Hello, terminal');
		setTimeout(() => {
			this.writeEmitter.fire('Goodbye!');
			this.exitEmitter.fire(0);
		}, 3000);
	}

	shutdown() {
		console.log("Shutting down");
	}

	input(data: string): void {
		console.log('There was input');
	}

	setDimensions?(dimensions: vscode.TerminalDimensions): void {
		console.log('Dims changed');
	}
}

I've added the start in the PR: 83c08e4