[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.

layouts/community/ergodox/osx_neo2/layers.h

@@ -0,0 +1,9 @@
+enum layers {
+  NEO_1,  // layer_0
+  NEO_3,  // layer_1
+  NEO_4,  // layer_2
+  NEO_5,  // layer_3
+  NEO_6,  // layer_4
+  US_1,   // layer_5
+  FKEYS   // layer_6
+};