targets: add support for Pimoroni Tufty 2350 board

Add a tufty2350 target and board file for the Pimoroni Tufty 2350,
an RP2350B board with a parallel ST7789 LCD and a CYW43439 wireless
chip. The target inherits rp2350b and sets a 16 MB flash size, like
the existing pga2350 and pico-plus2 targets.

Pin names come from Pimoroni's published pins.csv for the board:
https://github.com/pimoroni/tufty2350/blob/main/board/pins.csv

targets/tufty2350.json

@@ -0,0 +1,8 @@
+{
+    "inherits": ["rp2350b"],
+    "build-tags": ["tufty2350", "cyw43439"],
+    "default-stack-size": 8192,
+    "ldflags": [
+        "--defsym=__flash_size=16M"
+    ]
+}