Refactor shell extensions

- Use symlinks instead of generating the full content
- Update fish_user_paths and remove ~/.config/fish/functions/fzf.fish
- Create wrapper script for fzf when Ruby version and use it instead of
  exported function not to break fzf-tmux

fzf

@@ -623,7 +623,8 @@ class FZF
   def start_reader
     stream =
       if @source.tty?
-        if default_command = ENV['FZF_DEFAULT_COMMAND']
+        default_command = ENV['FZF_DEFAULT_COMMAND']
+        if default_command && !default_command.empty?
           IO.popen(default_command)
         elsif !`which find`.empty?
           IO.popen("find * -path '*/\\.*' -prune -o -type f -print -o -type l -print 2> /dev/null")