feat(boards): Add numpad layouts

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.

app/dts/layouts/common/numpad/position_map.dtsi

@@ -0,0 +1,7 @@
+/ {
+    layouts_common_numpad_position_map: layouts_common_numpad_position_map {
+        compatible = "zmk,physical-layout-position-map";
+
+        complete;
+    };
+};