Correctly Group Drag-and-drop for Undo

### Description Moves the `endUndoGrouping` call to the end of the actual editing operation when dropping text. Makes it so only one undo command is needed to undo a drag-and-drop operation. ### Related Issues * closes #99 ### Checklist - [x] I read and understood the [contributing guide](https://github.com/CodeEditApp/CodeEdit/blob/main/CONTRIBUTING.md) as well as the [code of conduct](https://github.com/CodeEditApp/CodeEdit/blob/main/CODE_OF_CONDUCT.md) - [x] The issues this PR addresses are related to each other - [x] My changes generate no new warnings - [x] My code builds and runs on my machine - [x] My changes are all related to the related issue above - [x] I documented my code ### Screenshots https://github.com/user-attachments/assets/35952874-6d4d-4702-b149-f06b9ffb5226