Allow extension to provide callback functions as tasks

[GabeDeBacker]

Our build environment is quite complex and offers more that just console output. For example, information can be obtained by listening to event tracing providers.

As such, our VSCode extension communicates with an out of process server (using JSON-RPC) to perform the build actions in our environment. The extension exposes commands that users must specifically invoke to perform a build action.

The desire is to integrate with VSCode's task infrastructure and have the task infrastructure execute an extension callback rather than having to launch a process.

This way, users can use VSCode's user interface as it is currently designed rather than have to locate our build commands.

[dbaeumer]

@Tyriar, @alexr00 and I had a discussion about that yesterday and we agreed on the following design / API / implementation.

• we add a method resolveTerminalRenderer to ExtHostTerminalService which takes a terminal ID.
• when a custom task is executed the task system sends the terminal ID over to the extension host and that id is then used to resolve a terminal renderer via the new method resolveTerminalRenderer
• the task API will expose a TerminalRenderer interface. If extensions start to call API on the renderer which tasks wants to control (for example show / hide) we can proxy the renderer / terminal and provide our own implementation.

The final custom task API will then look like this:

	/**
	 * Class used to execute an extension callback as a task.
	 */
	export class CustomTaskExecution {
		/**
		 * @param callback The callback that will be called when the task taking this execution is executed.
		 */
		constructor(callback: (renderer: TerminalRenderer, cancellationToken: CancellationToken, thisArg?: any) => Thenable<number | undefined>);
		/**
		 * The callback used to execute the task.
		 */
		callback: (renderer: TerminalRenderer, cancellationToken: CancellationToken, thisArg?: any) => Thenable<number | undefined>;
	}

I also changes the return value to Thenable<number | undefined> to be able to signal an error code from a started process.

[dbaeumer]

@jrieken above a new proposal for the CustomTask API.