feat(columns): per-character permission column highlights (#375)

Problem: the permissions column rendered as a monolithic unstyled
string, making it hard to scan `rwx` bits at a glance.

Solution: add per-character highlight groups for permission characters
following the `eza`/`lsd` convention. All groups link to standard
Neovim highlights so every colorscheme works out of the box.

lua/oil/adapters/ssh.lua

@@ -118,7 +118,11 @@ local ssh_columns = {}
 ssh_columns.permissions = {
   render = function(entry, conf)
     local meta = entry[FIELD_META]
-    return meta and permissions.mode_to_str(meta.mode)
+    if not meta then
+      return
+    end
+    local str = permissions.mode_to_str(meta.mode)
+    return { str, permissions.mode_to_highlights(str) }
   end,
 
   parse = function(line, conf)