root/admin prompt is red now (#5836)

I really miss bash's visual way of signalising root, i.e. blue: user, red: root

So I brought it to nushell (since you've added `is-admin` the code is fully portable and easily-readable) and hope you'll like it

docs/sample_config/default_env.nu

@@ -1,7 +1,11 @@
 # Nushell Environment Config File
 
 def create_left_prompt [] {
-    let path_segment = ($env.PWD)
+    let path_segment = if (is-admin) {
+        $"(ansi red_bold)($env.PWD)"
+    } else {
+        $"(ansi green_bold)($env.PWD)"
+    }
 
     $path_segment
 }