RTE plain text mentions as pills (#10852)

* insert mentions as links styled as pills

* post merge fix and update test

* update comments, move typeguard out

* create a text node instead of setting innerText

* update test

* update test

* fix broken cypress test, remove .only

* make it able to deal with inserting in middle of blank lines

* update comment

* fix strict null error

* use typeguard

* avoid implicit truth check

* add hook tests

* add comment

* Update test/components/views/rooms/wysiwyg_composer/hooks/usePlainTextListeners-test.tsx

Co-authored-by: Andy Balaam <andy.balaam@matrix.org>

---------

Co-authored-by: Andy Balaam <andy.balaam@matrix.org>

src/Typeguards.ts

@@ -17,3 +17,7 @@ limitations under the License.
 export function isNotNull<T>(arg: T): arg is Exclude<T, null> {
     return arg !== null;
 }
+
+export function isNotUndefined<T>(arg: T): arg is Exclude<T, undefined> {
+    return arg !== undefined;
+}