Add embedder API for creating simple readonly terminals

[Tyriar]

cc @osortega

I tested by adding this into BrowserMain:

const onDidWriteEmitter = new Emitter<string>();
embedderTerminalService.createTerminal({
	name: 'Building Codespace',
	pty: {
		onDidWrite: onDidWriteEmitter.event,
		open() {
			onDidWriteEmitter.fire('Hello world!\r\n');
			setInterval(() => onDidWriteEmitter.fire('\r\nwork...'), 1000);
		},
		close() { },
	}
});