[BUG] Can't map preview page up/down to pgup/pgdn keys

[danmou]

Describe the bug
Mapping pgup to builtin: pageUp and pgdn to builtin: pageDown has no effect, the keys do nothing. Mapping them to something else works, so the problem is not in receiving the key events.

To Reproduce

1. Add this to your config:

keybindings:
  universal:
    - key: pgup
      builtin: pageUp
    - key: pgdn
      builtin: pageDown

2. Open gh dash and try pressing pgup/pgdown

Expected behavior
The preview pane scrolls up and down

Desktop (please complete the following information):

• OS: Manjaro Linux

Side note
It was difficult to find the list of supported keys and their string representations. In the end I tracked it down here by looking through the code.