Unable to drag and drop text to the web app

It was reported in 3816060-zd-woothemes that in the previous version of the web app, they were able to drag and drop text from the other apps. However, the latest version of the web app doesn't support this feature anymore.

### Expected

I would expect to be able to drag and drop text to the web app.

### Observed

The text doesn't get copied to the web app with drag and drop.

### Reproduced

1. Highlight some text on another app (e.g. Firefox browser)
2. Drag the text to the web app's note editing area
3. See that the text doesn't get copied to the web app

**Screen recording when tested on https://app.simplenote.com/old**


**Screen recording when tested on https://app.simplenote.com**


### Where did you see the bug

- System Make: iMac
- System Model: iMac (Retina 5K, 27-inch, 2019)
- OS: macOS
- OS version: Big Sur 11.2.3 (20D91)
- Browser (if applicable): Google Chrome
- Browser version (if applicable): Version 89.0.4389.82 (Official Build) (x86_64)
- Simplenote app version: https://app.simplenote.com (2.7.1)

Start by reproducing the issue in the Simplenote web app's note editing area: drag highlighted text from another app into it and compare the current app with the old app shown in the recordings. Trace the editor's drag-and-drop handling, then verify that dragged text is copied into the note without breaking normal editing.

Written by the indexing model from the issue text.