feat(icon): add opt-in filetype detection via file contents

Problem: files without standard extensions (e.g. scripts with
shebangs, Makefile, Dockerfile) got incorrect or default icons since
icon providers match by filename or extension only.

Solution: add use_slow_filetype_detection option to the icon column
config. When enabled, reads the first 16 lines of each file and
passes them to vim.filetype.match for content-based detection. The
detected filetype is forwarded to mini.icons or nvim-web-devicons as
a trailing parameter, preserving backwards compatibility with
existing icon provider implementations.

Based on: stevearc/oil.nvim#618

doc/oil.txt

@@ -438,6 +438,10 @@ icon                                                                 *column-ico
       {directory}    `string` Icon for directories
       {add_padding}  `boolean` Set to false to remove the extra whitespace after
                      the icon
+      {use_slow_filetype_detection} `boolean` Set to true to detect filetypes
+                     by reading the first lines of each file (e.g. shebangs).
+                     This improves icon accuracy for extensionless scripts but
+                     performs synchronous I/O per file per render.
 
 size                                                                 *column-size*
     Adapters: files, ssh, s3