Fixes microsoft/monaco-editor#2000

By default the "go to defintion" action in the Monaco editor does nothing if the definition is in another model, or opens a peek window when multiple definitions are found. This default is a good thing because just changing the underlying model would be very detrimental when embedding the editor. However, there are use-cases where you maybe want to control what happens when you go to a definition. For example, when we embed the Monaco editor we would like to open a new tab to the source code containing the definition for certain models but not all of them.

This PR adds a new function monaco.editor.registerEditorOpener that lets the embedder intercept such open operations and handle those themselves. By using a boolean return value the embedder can indicate if the resource was handled or the editor should fallback to the default. The function returns an IDisposable instance that can be used to unregister the handler again. This interception can be tested in the code below by e.g. Ctrl-clicking on the alert function. By intercepting the open operation you can also choose to change the underlying editor model yourself if that makes sense in your application (see linked issue).

Edit: I decided to remove a second commit because it didn't work properly in some cases. This reduces the PR to just the new method to intercept the editor open operation.

Monaco Playground Code:

const text = `function hello() {
	alert('Hello world!');
	Date.now();
}`;

// Hover on each property to see its docs!
const editor = monaco.editor.create(document.getElementById("container"), {
	value: text,
	language: "javascript",
	automaticLayout: true,
});

monaco.editor.registerEditorOpener({
	openCodeEditor(source, resource, selectionOrPosition) {
		console.log(source, resource, selectionOrPosition);
		if (resource.path === "/lib.dom.d.ts") {
			// simulate openening a new browser tab for our own type (open definition of alert)
			console.log(`Opened definition for ${resource}`);

			// alternatively set model directly in the editor if you have your own tab/navigation implementation
			// const model = monaco.editor.getModel(resource);
			// editor.setModel(model);
			// if (monaco.Range.isIRange(selectionOrPosition)) {
			// 	editor.revealRangeInCenterIfOutsideViewport(selectionOrPosition);
			// 	editor.setSelection(selectionOrPosition);
			// } else {
			// 	editor.revealPositionInCenterIfOutsideViewport(selectionOrPosition);
			// 	editor.setPosition(selectionOrPosition);
			// }

			return true;
		}
		return false;
	}
});