Add 60_ansi_split_bs_rshift layout to DZ60, with a new keymap for the same (#6096)

Add 60_ansi_split_bs_rshift layout to DZ60, with a new keymap for the same (#6096)

* Add 60_ansi_split_bs_rshift layout to DZ60

I know there's already a lot of DZ60 layout macros, and #4668 suggests
they should be refactored at some point, but since this is one of the
standard layouts already in QMK that this PCB supports, I figured it was
okay to add so that DZ60 keyboards can share this layout with other
keyboards.

* New 60% ANSI split backspace/right-shift layout

I'm using this on a DZ60, but it should work fine on most 60% PCBs. It's
basically a HHKB layout with a standard ANSI bottom row (3x 1.25U mods,
6.25U spacebar, 4x 1.25U mods).