run command when changing directories

Created by: aeosynth

fish functions can trigger when variables (eg PWD) are changed; i use this to track my most used directories. shadowing cd isn't enough, as that wouldn't trigger on implicit directory changing.

https://fishshell.com/docs/current/commands.html#function

-v or --on-variable VARIABLE_NAME tells fish to run this function when the variable VARIABLE_NAME changes value.