Pasting text into tag editor erases the existing tag text

### Expected
When the tag editor is focused and contains some text, pasting new text from the clipboard should append the pasted text to existing text.

### Observed
Instead of being appended, the text replaces the existing one:

https://user-images.githubusercontent.com/73365754/128499755-05d1aced-ebe6-4ba8-953e-799496e7724b.mov

### Reproduced
1. Create a new note
2. Place `text1` into the clipboard (just a single word for simplicity)
3. Start entering `text2` in tag editor, and then paste `text1` ~> this action will replace `text2` with `text1`, instead of appending: `text2text1`

### Where did you see the bug
- OS version: Widows 10 Home on Parallels Desktop
- Simplenote app version: desktop 2.16.0-beta1

Start by reproducing the behavior in the tag editor using the listed steps: enter text2, then paste text1. Locate the tag editor entry point and trace how paste handles the focused text; done means the existing text remains and the pasted text is appended, as in text2text1.

Written by the indexing model from the issue text.