Desktop: Accessibility: Prevent overwrite mode toggle shortcut from conflicting with screen reader shortcuts

[personalizedrefrigerator]

Summary

On Linux, the Orca screen reader's default modifier key is insert. Before this pull request, pressing insert as part of a screen reader shortcut (e.g. insert-h), entered overwrite mode.

This pull request:

• Prevents insert from entering overwrite mode if another key was pressed before releasing insert.
• To allow this check to work, enters/exits insert mode on keyup rather than on keydown.
  • A custom keyUpHandler CodeMirror extension was created to allow this.

Testing plan

This pull request adds a new overwriteModeExtension automated test that verifies that the following process does not enable insert mode:

1. Pressing insert.
2. Pressing and releasing some other key.
3. Releasing insert.

Additionally, on Fedora 41:

1. Enable the Orca screen reader.
2. Open Joplin and open a note in the Markdown editor.
3. Press insert.
   • With Orca enabled (at least with my desktop environment), the onkeyup event handler is not invoked when the user releases insert (though the onkeydown handler is on key down). With Orca disabled, both onkeydown and onkeyup are received.
4. Verify that insert mode has not been enabled.
5. Press insert twice quickly.
6. Verify that overwrite mode is enabled.
7. Press escape.
8. Verify that overwrite mode is disabled.
9. Disable Orca and restart Joplin.
10. Open a note.
11. Press insert.
12. Verify that overwrite mode is enabled.

[personalizedrefrigerator]

See #11441