Up function (#199)

* Add up function.

* Only works in config.nu

* Simplify script, using nushell updir notation.

filesystem/up.nu

@@ -0,0 +1,10 @@
+def up_inner [limit: int] {
+  (for $e in 0..$limit { "." } | str collect)
+}
+
+# Go up a number of directories
+def-env up [
+    limit: int # The number of directories to go up
+  ] {
+    cd (up_inner $limit)
+}