Create (empty) configuration directories and files on first run

[Tadaboody]

fish, version 3.1.0 (running OS X, tested on a ubuntu container to see the setup behaviour)

Completion scripts should go in ~/.config/fish/completions but that dir does not exist when starting to use fish, I think the process could be streamlined if it was created at setup (like .config/fish/ already is)

[IlanCosman]

Neither is the conf.d directory, or the functions directory, or the config.fish file. It might help new users if those were all created, blank of course.

[zanchey]

This can't be done on install but rather on first run, but I think it's a reasonable idea - the base directories already get created anyway. Probably this could be done in __fish_config_interactive?

[krobelus]

Yeah it seems fine. Many people might never use those directories but they don't hurt.

Compare to git init which creates plenty of example hooks in .git/hooks/.