don't put cursor position in NewlinePart after adding it

You can't append to it anyway, so mark it uneditable and skip
uneditable parts if that's where an edit ended up.

This has the added advantage that if there is text after a newly
insert pill, the cursor will be put just before it rather than
in the pill, after the last character.

src/editor/model.js

@@ -88,7 +88,8 @@ export default class EditorModel {
         }
         this._mergeAdjacentParts();
         const caretOffset = diff.at - removedOffsetDecrease + addedLen;
-        const newPosition = this._positionForOffset(caretOffset, true);
+        let newPosition = this._positionForOffset(caretOffset, true);
+        newPosition = newPosition.skipUneditableParts(this._parts);
         this._setActivePart(newPosition);
         this._updateCallback(newPosition);
     }
@@ -261,4 +262,13 @@ class DocumentPosition {
     get offset() {
         return this._offset;
     }
+
+    skipUneditableParts(parts) {
+        const part = parts[this.index];
+        if (part && !part.canEdit) {
+            return new DocumentPosition(this.index + 1, 0);
+        } else {
+            return this;
+        }
+    }
 }