Color the default prompt host if running via SSH

This is part of our (well, my) quest to spice up the default prompt.

In this case we color the host if $SSH_TTY is set, which is easy to
detect and helps draw attention to the host.

See #6398.
See #6375.

share/functions/__fish_config_interactive.fish

@@ -58,6 +58,7 @@ function __fish_config_interactive -d "Initializations that should be performed
         __init_uvar fish_color_autosuggestion 555 brblack
         __init_uvar fish_color_user brgreen
         __init_uvar fish_color_host normal
+        __init_uvar fish_color_host_remote yellow
         __init_uvar fish_color_valid_path --underline
         __init_uvar fish_color_status red