Change Hover#range to be optional

The docs of `Hover#range` state:

> When missing, the editor will use the range at the current position or the current position itself.

This implies that the `range` field should be optional as it can be missing.

src/vs/editor/common/modes.ts

@@ -230,7 +230,7 @@ export interface Hover {
 	 * editor will use the range at the current position or the
 	 * current position itself.
 	 */
-	range: IRange;
+	range?: IRange;
 }
 
 /**