fix: Add delay to DemuxKeyMatrix

[patricksurry]

I'm using a nullbitsco nibble which has a 16x5 matrix with the multiplexed via two '138 3:8 decoders.
With no delay the DemuxKeyMatrix scanner was giving phantom key strokes on almost every key press, where (r,c) is
pressed but (r,c), (r+1,c) and sometimes (r+2,c) register. I tried the same 1us delay as the basic KeyMatrix but
still saw echoes, especially where many bits change in the demux input. The qmk implementation uses a 5us delay
which also works here.

In passing this class would be more flexible if it supported the same columns_to_anodes parameter as KeyMatrix, and perhaps a way to transpose rows and columns. For example with the nibble it's actually the columns that are multiplexed not the rows but I can work around by pretending they're rows and transposing back to the keymap.

[dhalbert]

This looks fine! Would you like to add your proposed additions to the API?