update last caret from update callback instead of input event

many editor updates are not caused by an input event, so
the last caret wasn't always up to date.

Updating the caret from the update callback ensures that every
time the editor contents is changed, the last caret is updated.

src/editor/position.js

@@ -120,4 +120,17 @@ export default class DocumentPosition {
         const atEnd = offset >= lastPart.text.length;
         return new DocumentOffset(offset, atEnd);
     }
+
+    isAtEnd(model) {
+        if (model.parts.length === 0) {
+            return true;
+        }
+        const lastPartIdx = model.parts.length - 1;
+        const lastPart = model.parts[lastPartIdx];
+        return this.index === lastPartIdx && this.offset === lastPart.text.length;
+    }
+
+    isAtStart() {
+        return this.index === 0 && this.offset === 0;
+    }
 }