Support F20–F24

[smammy]

macOS supports fkeys up to F24, but Rectangle only supports F1–F19. (I remap certain fn+key combinations to high-numbered fkeys as a workaround for #288.)

[rxhanson]

Thanks for reporting. As mentioned in that linked issue, since I have ownership of that MASShortcut fork, I'll see what I can do to allow up to F24.

[smammy]

Well, here's F20 support, for what it's worth: rxhanson/MASShortcut#1

[smammy]

Although apparently F21–F24 are cursed.