fix multi-character insertion

When multiple characters are inserted atomically (eg in a paste),
Prosemirror generates a step with a single insertion string.
But Automerge expects us to give it one character at a time,
so we need to split the character up ourselves.

See for more context:
automerge/automerge#326

src/index.ts

@@ -128,7 +128,7 @@ function applyTransaction(
                     if (doc.content.insertAt) {
                         doc.content.insertAt(
                             contentPosFromProsemirrorPos(step.from),
-                            insertedContent,
+                            ...insertedContent.split(""),
                         )
                     }
                 })