feat(columns): per-character permission column highlights (#375) (#146)

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/files.lua

@@ -100,7 +100,8 @@ if not fs.is_windows then
       if not stat then
         return columns.EMPTY
       end
-      return permissions.mode_to_str(stat.mode)
+      local str = permissions.mode_to_str(stat.mode)
+      return { str, permissions.mode_to_highlights(str) }
     end,
 
     parse = function(line, conf)