Add "smoke test" sketch compilation CI workflow for MuxTO firmware

[per1234]

On every push and pull request that affects relevant files, and periodically, compile the Nano Every's MuxTO firmware sketch.

The binary is saved as a workflow artifact to make it available for beta testers who might prefer flashing it directly over setting up the toolchain needed to compile it locally.

Due to the standard output from Arduino CLI not providing the correct size data, a bespoke check is done for whether the compiled binary will fit in the available 12 kB of memory. If not, the workflow run fails.

On creation or commit to a pull request, a report of the resulting change in memory usage of the sketch will be commented to the PR thread.

Related: https://github.com/arduino/ArduinoCore-megaavr/issues/103

[github-actions[bot]]

Memory usage change @ d39a5ab3adecddc6d3b80ca212d1dbed805f77a7

Board	flash	%	RAM for global variables	%
arduino:samd:muxto:float=default,config=enabled,clock=internal_usb,timer=timer_732Hz,bootloader=4kb,serial=two_uart,usb=cdc	0 - 0	0.0 - 0.0	N/A	N/A

Click for full report table

Board|firmwares/MuxTO
flash|%|firmwares/MuxTO
RAM for global variables|% -|-|-|-|- arduino:samd:muxto:float=default,config=enabled,clock=internal_usb,timer=timer_732Hz,bootloader=4kb,serial=two_uart,usb=cdc|0|0.0|N/A|N/A

Click for full report CSV

``` Board,firmwares/MuxTO
flash,%,firmwares/MuxTO
RAM for global variables,% "arduino:samd:muxto:float=default,config=enabled,clock=internal_usb,timer=timer_732Hz,bootloader=4kb,serial=two_uart,usb=cdc",0,0.0,N/A,N/A ```

[mcuee]

Looks like this never runs successfully. https://github.com/arduino/ArduinoCore-megaavr/actions/workflows/compile-muxto.yml