feat(icons): add direct nonicons.nvim icon provider

Problem: nonicons.nvim exposes a public get_icon/get_icon_by_filetype
API, but oil.nvim can only use nonicons glyphs indirectly through the
devicons monkey-patch. This couples oil to devicons even when nonicons
is available standalone.

Solution: add a nonicons provider in get_icon_provider() between
mini.icons and devicons. Feature-gated on nonicons.get_icon existing
so old nonicons versions fall through to devicons. Uses OilDirIcon
and OilFileIcon highlight groups.

lua/oil/util.lua

@@ -945,6 +945,28 @@ M.get_icon_provider = function()
     end
   end
 
+  local has_nonicons, nonicons = pcall(require, 'nonicons')
+  if has_nonicons and nonicons.get_icon then
+    return function(type, name, conf, ft)
+      if type == 'directory' then
+        local icon = nonicons.get('file-directory')
+        return icon or (conf and conf.directory or ''), 'OilDirIcon'
+      end
+      if ft then
+        local ft_icon = nonicons.get_icon_by_filetype(ft)
+        if ft_icon then
+          return ft_icon, 'OilFileIcon'
+        end
+      end
+      local icon = nonicons.get_icon(name)
+      if icon then
+        return icon, 'OilFileIcon'
+      end
+      local fallback = nonicons.get('file')
+      return fallback or (conf and conf.default_file or ''), 'OilFileIcon'
+    end
+  end
+
   local has_devicons, devicons = pcall(require, 'nvim-web-devicons')
   if has_devicons then
     return function(type, name, conf, ft)