Change WidgetType.updateDOM from type to this

With `this` TypeScript understands that it's the same type as the subclass.

Author: not-my-profile

src/decoration.ts

@@ -121,7 +121,7 @@ export abstract class WidgetType {
   /// true to indicate that it could update, false to indicate it
   /// couldn't (in which case the widget will be redrawn). The default
   /// implementation just returns false.
-  updateDOM(dom: HTMLElement, view: EditorView, from: WidgetType): boolean { return false }
+  updateDOM(dom: HTMLElement, view: EditorView, from: this): boolean { return false }
 
   /// @internal
   compare(other: WidgetType): boolean {