Extending selection using cursor keys is incomplete

Currently extending selection using cursor keys is incomplete.

Steps to reproduce:

1. Create several blocks (preferably paragraphs containing text)
2. Navigate to one of the centermost blocks
3. Try extending the current select upward using either Ctrl+Shift+Up or Ctrl+Shift+PgUp
4. Try extending the current select downward using either Ctrl+Shift+Down or Ctrl+Shift+PgDn

Expected behavior:

- Ctrl+Shift+Up would extend the selection to the start of document
- Ctrl+Shift+PgUp would extend the selection to up by one page
- Ctrl+Shift+Down would extend the selection to end of document
- Ctrl+Shift+PgDn would extend the selection down by one page

Device, Browser, OS:

Windows 10, Chromium Edge

Editor.js version:

Tested with 2.18 and master current.

Plugins you use with their versions:

Editor without custom plugins.