refactor(main): move file operation functions to new source (#2124)

src/file_handler.h

@@ -0,0 +1,14 @@
+/**
+ * @file file_handler.h
+ * @brief Header file for file handling functions.
+ */
+#pragma once
+
+#include <string>
+
+namespace file_handler {
+  std::string
+  read_file(const char *path);
+  int
+  write_file(const char *path, const std::string_view &contents);
+}  // namespace file_handler