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

scripts/generate.py

@@ -154,6 +154,11 @@ COL_DEFS = [
                 "boolean",
                 "Set to false to remove the extra whitespace after the icon",
             ),
+            LuaParam(
+                "use_slow_filetype_detection",
+                "boolean",
+                "Set to true to detect filetypes by reading the first lines of each file (e.g. shebangs).",
+            ),
         ],
     ),
     ColumnDef("size", "files, ssh, s3", False, True, "The size of the file", UNIVERSAL + []),