Add page up & down key inputs

[CamKem]

This pull request includes updates to the Key class in add new key constants for navigation.

Key constants added:

• PAGE_UP constant with the value "\e[5~"
• PAGE_DOWN constant with the value "\e[6~"

These escape sequences align with the most common VT100/ANSI emulation in modern terminal environments and are particularly useful for handling Page Up/Down navigation. For reference please GNU documentation on input translation in emulation of the VT100

[jessarcher]

Thanks for the PR! It feels a little weird to include these constants but not use them internally, but it's cool that folks are extending Prompts in all sorts of interesting ways and it's nice to have a single source of truth for this stuff.