feat(boards): Add numpad layouts

[joelspadin]

Added physical layouts for the following variants of numpads:

• With and without extra top row
• 2U plus key or 1U plus and backspace keys
• 2U 0 key or 1U 0 and 00 keys
• Full 1U grid/macropad layout

Other layouts exist, such as "southpaw" horizontally mirrored layouts, and layouts with a fifth column, but those seem to be much less common. There are a lot of layouts here already, so we can add the less common ones later if anyone asks for them.

[nmunnich]

LGTM