♻️ Extract file backend into utils subpackage

src/utils/file_backend_stub.mbt

@@ -0,0 +1,51 @@
+pub struct FileHandle {
+  path : String
+}
+
+pub fn open_file_handle_internal(path : String, append : Bool) -> FileHandle? {
+  ignore(append)
+  ignore(path)
+  let _unused : FileHandle = { path: "" }
+  ignore(_unused)
+  None
+}
+
+pub fn write_file_handle_internal(handle : FileHandle, content : String) -> Bool {
+  ignore(handle)
+  ignore(content)
+  false
+}
+
+pub fn flush_file_handle_internal(handle : FileHandle) -> Bool {
+  ignore(handle)
+  false
+}
+
+pub fn close_file_handle_internal(handle : FileHandle) -> Bool {
+  ignore(handle)
+  false
+}
+
+pub fn file_size_internal(handle : FileHandle) -> Int {
+  ignore(handle)
+  0
+}
+
+pub fn rename_file_internal(from_path : String, to_path : String) -> Bool {
+  ignore(from_path)
+  ignore(to_path)
+  false
+}
+
+pub fn remove_file_internal(path : String) -> Bool {
+  ignore(path)
+  false
+}
+
+pub fn string_byte_length_internal(content : String) -> Int {
+  content.length()
+}
+
+pub fn native_files_supported_internal() -> Bool {
+  false
+}