Allow getWorker function to return a promise

[CGNonofr]

This PR fixes microsoft/monaco-editor#2986

[ghost]

All CLA requirements met.

[hediet]

Thank you!

Reference:

vscode/src/vs/base/browser/defaultWorkerFactory.ts

Lines 11 to 16 in 23aa7ad

	function getWorker(label: string): Worker | Promise<Worker> {
	// Option for hosts to overwrite the worker script (used in the standalone editor)
	if (globals.MonacoEnvironment) {
	if (typeof globals.MonacoEnvironment.getWorker === 'function') {
	return globals.MonacoEnvironment.getWorker('workerMain.js', label);
	}

Since this is a type declaration change that only affects monaco, we can include this fix in this milestone.