feat: Add posix_pro_micro and posix_seeed_xiao shields

* Make it easier to test native builds of our shields by adding
  `posix_pro_micro` and `posix_seeed_xiao` so you can build posix
  target of, e.g. `corne_left` for testing ZMK Studio.

app/boards/shields/posix_seeed_xiao/Kconfig.defconfig

@@ -0,0 +1,9 @@
+# Copyright (c) 2024 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+if SHIELD_POSIX_SEEED_XIAO
+
+config ZMK_STUDIO
+    select UART_NATIVE_POSIX_PORT_1_ENABLE
+
+endif