Create functions, completions, and conf.d in $fisher_path; fix #598

A cleaner approach would be to move fisher.fish to functions
but this would break older versions of fisher that hardcoded
the raw URL to the fisher.fish file for upgrades.

fisher.fish

@@ -97,6 +97,8 @@ function fisher -a cmd -d "fish plugin manager"
             end
             wait $pid_list 2>/dev/null
 
+            command mkdir -p $fisher_path/{functions,completions,conf.d}
+
             for plugin in $install_plugins $update_plugins
                 set -l data $fisher_data/$plugin
                 test -e $plugin && set data $fisher_data/@$USER/(string replace --all --regex '^.*/' "" $plugin)