microbit-v2: add support for S113 SoftDevice

This currently doesn't work with `tinygo flash` yet (even with
`-programmer=openocd`), you can use pyocd instead. For example, from the
Bluetooth package:

    tinygo build -o test.hex -target=microbit-v2-s113v7 ./examples/advertisement/
    pyocd flash --target=nrf52 test.hex

I intend to add support for pyocd to work around this issue, so that a simple
`tinygo flash` suffices.

targets/microbit-v2-s113v7.json

@@ -0,0 +1,3 @@
+{
+	"inherits": ["microbit-v2", "nrf52833-s113v7"]
+}