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).

layouts/community/60_ansi_split_bs_rshift/bcat/readme.md

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+# bcat's 60% ANSI split backspace/right-shift layout
+
+This is a hybrid of an HHKB layout and a standard ANSI bottom row. It's nice if
+you want to fill out a 60% case with no blockers, or just really want a 6.25U
+spacebar. The arrow and navigation keys match a standard HHKB layout (using the
+Fn key next to the right shift key), but the media keys are centered around the
+WASD cluster instead of the usual HHKB layout. Additionally, the redundant Fn
+key on the bottom row activates an adjust layer to control RGB underglow.
+
+## Default layer
+
+![Default layer layout](https://i.imgur.com/HM0115k.png)
+
+## Function layer
+
+![Function layer layout](https://i.imgur.com/8dr5ktH.png)
+
+## Adjust layer
+
+![Adjust layer layout](https://i.imgur.com/78PBNt6.png)