Add yay AUR helper functions

[py-crash]

A new file functions/yay.fish is created, which contains the functions for the yay AUR helper. Only one file is preferred instead of the typical one file per function for performance reasons. Since fish needs to check if yay is installed before loading the functions, one file is used so the if statement is only evaluated once. Multiple files will ake the process much slower for loading the shell. The functions/README.md file is also updated to document these new functions. Only a short description is provided since the added functions are pretty much self-explanatory.