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/files/permissions.lua

@@ -25,6 +25,31 @@ M.mode_to_str = function(mode)
     .. perm_to_str(bit.band(extra, 1) ~= 0 and 't', mode)
 end
 
+local char_hl = {
+  r = 'OilPermissionRead',
+  w = 'OilPermissionWrite',
+  x = 'OilPermissionExec',
+  s = 'OilPermissionSetuid',
+  S = 'OilPermissionSetuid',
+  t = 'OilPermissionSticky',
+  T = 'OilPermissionSticky',
+  ['-'] = 'OilPermissionNone',
+}
+
+---@param str string
+---@return table[]
+M.mode_to_highlights = function(str)
+  local highlights = {}
+  for i = 1, #str do
+    local c = str:sub(i, i)
+    local hl = char_hl[c]
+    if hl then
+      table.insert(highlights, { hl, i - 1, i })
+    end
+  end
+  return highlights
+end
+
 ---@param mode integer
 ---@return string
 M.mode_to_octal_str = function(mode)