Improve switching between rich and plain editing modes (#9776)

* allows switching between modes that retains formatting
* updates rich text composer dependency to 0.13.0 (@matrix-org/matrix-wysiwyg)
* improves handling of enter keypresses when ctrlEnterTosend setting is true in plain text editor
* changes the message event content when using the new editor
* adds tests for the changes to the plain text editor

package.json

@@ -57,7 +57,7 @@
     "dependencies": {
         "@babel/runtime": "^7.12.5",
         "@matrix-org/analytics-events": "^0.3.0",
-        "@matrix-org/matrix-wysiwyg": "^0.11.0",
+        "@matrix-org/matrix-wysiwyg": "^0.13.0",
         "@matrix-org/react-sdk-module-api": "^0.0.3",
         "@sentry/browser": "^7.0.0",
         "@sentry/tracing": "^7.0.0",