Add monitor_serial to viewcrossmon and tagcrossmon

[Catskinner6]

I frequently dock my laptop with a dual monitor setup and have found that at times the monitor names change from say DP-2 adn DP-3 to DP-4 and DP-7. My understanding is this is random, depending on certain conditions in how the monitors are named.

This will cause my config for MangoWC to partially break, as I am using:

# View tags 1-6 on DP-6 -> DP-8
bind=SUPER,1,viewcrossmon,1,DP-8
bind=SUPER,2,viewcrossmon,2,DP-8
bind=SUPER,3,viewcrossmon,3,DP-8
bind=SUPER,4,viewcrossmon,4,DP-8
bind=SUPER,5,viewcrossmon,5,DP-8
bind=SUPER,6,viewcrossmon,6,DP-8

# View tags 7-9 on DP-7 -> DP-9
bind=SUPER,7,viewcrossmon,7,DP-9
bind=SUPER,8,viewcrossmon,8,DP-9
bind=SUPER,9,viewcrossmon,9,DP-9

viewcrossmon and tagcrossmon only take in "monitor_name", so the option for "monitor_serial" would be a better/more robust system for me so I don't have to worry about these monitor name changes from my dock.

[DreamMaoMao]

https://github.com/DreamMaoMao/mangowc/wiki#monitor_spec

support now.