feat: add OilFileIcon highlight group for fallback file icons

Problem: when nvim-web-devicons returns no highlight for an unrecognized
file extension, the icon gets nil for its highlight group. Directories
have OilDirIcon as a targetable fallback, but files have nothing.

Solution: define OilFileIcon (linked to nil, matching OilFile) and use
it as the fallback in the devicons provider when get_icon returns a nil
highlight. The mini.icons provider already returns highlights for all
files so only the devicons path needs this.

Resolves: stevearc/oil.nvim#690

lua/oil/util.lua

@@ -956,6 +956,7 @@ M.get_icon_provider = function()
           end
         end
         local icon, hl = devicons.get_icon(name)
+        hl = hl or "OilFileIcon"
         icon = icon or (conf and conf.default_file or "")
         return icon, hl
       end