Move `FunctionDeclImporter` to its own file.

PiperOrigin-RevId: 444186354
diff --git a/rs_bindings_from_cc/BUILD b/rs_bindings_from_cc/BUILD
index 40837d0..286ab10 100644
--- a/rs_bindings_from_cc/BUILD
+++ b/rs_bindings_from_cc/BUILD
@@ -166,6 +166,7 @@
         "//rs_bindings_from_cc/importers:class_template",
         "//rs_bindings_from_cc/importers:cxx_record",
         "//rs_bindings_from_cc/importers:enum",
+        "//rs_bindings_from_cc/importers:function",
         "//rs_bindings_from_cc/importers:function_template",
         "@absl//container:flat_hash_map",
         "@absl//container:flat_hash_set",