improve support for -S2

got myself an adafruit "qt py -S2" with tinyUF2 bootloader. So I wanted to get that working. And it does work :-).
FFT time = 10ms (instead of 2ms on classic ESP32)  is a very good result, also virtual USB runs much better now.

wled00/pin_manager.cpp

@@ -538,6 +538,7 @@ bool PinManagerClass::isPinOk(byte gpio, bool output)
     // 38 to 48 are for general use. Be careful about straping pins GPIO45 and GPIO46 - these may be pull-up or pulled-down on your board.
   #elif defined(CONFIG_IDF_TARGET_ESP32S2)
     // strapping pins: 0, 45 & 46
+    if (gpio > 18 && gpio < 21) return false;     // WLEDMM: 19 + 20 = USB HWCDC. Not recommended for other uses.
     if (gpio > 21 && gpio < 33) return false;     // 22 to 32: not connected + SPI FLASH
     // JTAG: GPIO39-42 are usually used for inline debugging
     // GPIO46 is input only and pulled down