[BUG]: The editor doesn't switch between INS/OVR modes

[nalsuriv]

Description

The editor always opens files in the 'Insert' mode, i.e., if one wants to add something to previously written text, they can place a cursor wherever they wish, and type in the missing text.

However, if one wants to replace the previously written text, pressing the [insert] button doesn't switch to the 'Overtype' mode (immediate character-for-character replacement), as it does in many plain-text editors.

To Reproduce

1. Create or open a text file in the application.
2. Write some text in the file.
3. Place the cursor anywhere before the end of the text.
4. Press the 'Insert' key on your keyboard.
5. Write some more text in the file.
6. Previous text does not get replaced.
7. Create or open a text file in another text editor (Kate, Notepad++, Geany, gedit etc).
8. Write some text in the file.
9. Place the cursor anywhere before the end of the text.
10. Press the 'Insert' key on your keyboard.
11. Write some more text in the file.
12. Previous text gets replaced with new text as you type it.
13. Press the 'Insert' key on your keyboard again.
14. Place the cursor anywhere before the end of the text.
15. Write some more text in the file.
16. Previous text does not get replaced.
17. Compare and contrast.

Expected behavior
The text editor needs to react to the pressing of the [Insert] key the same way other text editors behave (Kate, Notepadqq, etc.), and switch between INS/OVR modes accordingly.