feat(help): header-driven --help for listing presets

lD, lsr, lss, lstree, lt, ltr, lx and ld. None shadows a real binary, so
there is no disabled-fallback contract to honour, and `eza --help`
documented eza rather than the preset that is these functions' entire
content. First-arg-only interception leaves `lt -la --help` passing
through to eza untouched.

functions/lss.fish

@@ -17,6 +17,8 @@
 # EXAMPLE
 #   lss ~/downloads
 function lss --description 'Size-sorted listing'
+    __fish_help_header (status current-function) $argv; and return 0
+
     if which eza >/dev/null 2>&1
         eza --oneline --long --all --sort=size --icons --color=auto --hyperlink --color-scale=size --color-scale-mode=gradient $argv
     else if which lsd >/dev/null 2>&1