Fish config refactor

Move things into sub-files and sub-directories
Add ensure_path function to make sure stuff doesn't get spammed multiple
times into the PATH

fish/functions/ensure_path.fish

@@ -0,0 +1,7 @@
+function ensure_path --description "Ensures directories are in $PATH"
+    for pathdir in $argv
+        if not set -l index (contains -i $pathdir $PATH)
+            set -g -x PATH $pathdir $PATH
+        end
+    end
+end