Output selection to stdout if used in command substitution

[PatrickF1]

#104 (comment)

@PatrickF1 Not sure if you are interested, do you think it is favorable to output everything to stdout when executing a command substitution?

Currently all functions insert the output into the command line, which is fine for interactive usage, but not very interpolatable.

We can rewrite everything into something like:

if status is-command-substitution
    printf $output
else
    commandline --current-token --replace $output
end

That way people can reuse existing functions to achieve more niche usage, for example this feature request can become $EDITOR (__fzf_search_current_dir).

the idea is to make fzf.fish usable as just a library of functions for users to use to create their own key bindings.
Then people can do
bind [key] vim (__fzf_search_current_dir) or bind [key] git add (__fzf_search_git_status)

I think we could split the functions in two:

1. A base you can use in substitution (taking a token as optional parameter)
2. A wrapper of 1, that deals with what's necessary to make it run with a key binding and fetch the token