[Keymap] Neo2 for ErgoDox on MacOS US QWERTY / ABC Extended keymap (#5862)

[Keymap] Neo2 for ErgoDox on MacOS US QWERTY / ABC Extended keymap (#5862)

This is a Neo2 inspired layout that is meant to be fully usable on
MacOS when used with the default US QWERTY/ABC Extended keymap.

Neo2 layers 1-4 have been almost fully implemented in hardware.
Layers 5 and 6 (greek and mathematical symbols) have been left
out for now as most of them aren't available on the default
keymaps.

Layer toggling for layer 3 on the right hand side utilizes a
tap-toggle approach that is a combination of MO & LT macros.
This is required to allow sending Y when tapped, @ when tapped
while the SHIFT modifier is active and support momentarily
toggling the layer while the key is held.