Add C-j and C-k to keybinds for picker

[VuiMuich]

As discussed in matrix chat, add hard coded keybinds C-j for down and C-k for up in picker.

Closes #615.

[VuiMuich]

Would I need to add anything to the docs manually?

[archseer]

Yeah, at the end of keymap.md, you want to add them here:

helix/book/src/keymap.md

Lines 247 to 248 in cdfa0df

	| `Up`, `Ctrl-p` | Previous entry |
	| `Down`, `Ctrl-n` | Next entry |

[VuiMuich]

Thanks for merging!