[enhancement] Create block for the digipin multiplexer

[rmoesbergen]

We now have a digital pin multiplexer (8 bit) as a plug-in board. Some issues we found with this:

• It uses pin 0/1 for the address, which conflicts with Serial / USB / UART0. So if you use Serial at all, the multiplexer doesn't work bc the pins are uses/claimed by the UART.
• It doesn't have state, so only one pin at a time can be controlled.
• There's no PWM pin attached, so PWM doesn't work with this multiplexer

[rmoesbergen]

The block should have a pin number parameter/field that can be set with a variable (so the pin address can be set in a loop for example). It should give a warning icon on the block if it's used in combination with Serial IO.