kitsym4: Support the keeb.io Iris using the ProMicro<->ItsyBitsyM4 converter board

The thumb cluster maps a little goofy and I'd like to clean up the
keymap here - it's ENTIRELY NOT OBVIOUS how this works right now. Using
this keymap as an example, the physical layout of my thumb cluster is
actually:

Far left: Left
Next to the right: Right
Bottom right: Shift
Top right: MO(2)

You can see what this maps to in code, and it's not at all intuitive.

`swap_indicies`, which we already support, is useless here because,
unlike the Planck/Klaranck, these aren't 1:1 key swaps, but an entirely
custom mapping of columns. This will require something like QMK's
solution to fully custom (or at least partially custom) keymaps at a
core level, and isn't something I feel like tackling tonight
necessarily.

kmk/boards/kitsym4_iris.py

@@ -0,0 +1,19 @@
+import board
+
+from kmk.consts import DiodeOrientation
+from kmk.mcus.circuitpython_samd51 import Firmware as _Firmware
+from kmk.pins import Pin as P
+
+
+class Firmware(_Firmware):
+    # Pin mappings for converter board found at hardware/README.md
+    # QMK: MATRIX_COL_PINS { F6, F7, B1, B3, B2, B6 }
+    # QMK: MATRIX_ROW_PINS { D7, E6, B4, D2, D4 }
+    col_pins = (P.A2, P.A3, P.A4, P.A5, P.SCK, P.MOSI)
+    row_pins = (P.D11, P.D10, P.D9, P.RX, P.D13)
+    diode_orientation = DiodeOrientation.COLUMNS
+
+    split_flip = True
+    split_offsets = (6, 6, 6, 6, 6)
+    split_type = "UART"
+    uart_pin = board.SCL