Update to new version of slate

Lots of fixes here as a lot of the API has changed (eg. anchorKey /
offsetKey are now anchor.key and offset.key, and collapseFocusToThing
is moveFocusToThing).

Also changes the ref to a function (sorry for lumping this into the
same PR).

Hopefully will fix https://github.com/vector-im/riot-web/issues/7105

package.json

@@ -88,10 +88,10 @@
     "react-gemini-scrollbar": "matrix-org/react-gemini-scrollbar#5e97aef",
     "resize-observer-polyfill": "^1.5.0",
     "sanitize-html": "^1.18.4",
-    "slate": "0.34.7",
+    "slate": "^0.41.2",
     "slate-html-serializer": "^0.6.1",
     "slate-md-serializer": "matrix-org/slate-md-serializer#f7c4ad3",
-    "slate-react": "^0.12.4",
+    "slate-react": "^0.18.10",
     "text-encoding-utf-8": "^1.0.1",
     "url": "^0.11.0",
     "velocity-vector": "vector-im/velocity#059e3b2",