Fix regression around pasting links (#8537)

* Fix regression around pasting links

* Add tests

test/editor/mock.ts

@@ -18,6 +18,7 @@ import { Room, MatrixClient } from "matrix-js-sdk/src/matrix";
 
 import AutocompleteWrapperModel from "../../src/editor/autocomplete";
 import { PartCreator } from "../../src/editor/parts";
+import DocumentPosition from "../../src/editor/position";
 
 class MockAutoComplete {
     public _updateCallback;
@@ -78,11 +79,11 @@ export function createPartCreator(completions = []) {
 }
 
 export function createRenderer() {
-    const render = (c) => {
+    const render = (c: DocumentPosition) => {
         render.caret = c;
         render.count += 1;
     };
     render.count = 0;
-    render.caret = null;
+    render.caret = null as DocumentPosition;
     return render;
 }