Add M5 Stamp S3 support

[jpbelo]

The stamp s3 works fine with the multi s3 build, but the built in LED is always white full brightness. This basically duplicates the multi s3 configuration adding the neo pixel on pin 21. Maybe some configurations are not needed, but I tried to make as little change as needed compared to the multi s3.

Tested on a M5 Stamp S3 mounted on a flipper zero and works fine, and the LED displays the correct colors for the modes.

Still curious why the BT is disabled on the S3, but when I enable it the build breaks.

[kokein]

Hi ! Did You manage to M5Stamp full functionality?