Exposed _preview editor from references widget.

With the new internal API, one can have better typing when customizing
the peek editor from `monaco`.

Signed-off-by: Akos Kitta <kittaakos@typefox.io>

packages/monaco/src/typings/monaco/index.d.ts

@@ -987,7 +987,8 @@ declare module monaco.referenceSearch {
         focusOnReferenceTree(): void;
         focusOnPreviewEditor(): void;
         isPreviewEditorFocused(): boolean;
-        _tree: ReferenceTree
+        _tree: ReferenceTree;
+        _preview: ICodeEditor;
     }
 
     // it's used as return value for referenceWidget._tree