Split up main.c while keeping a single compilation unit

src/get_handler.c

@@ -0,0 +1,6 @@
+static enum MHD_Result handle_get(struct MHD_Connection *connection, SV url, SV base_path, SV username) {
+    (void)url;
+    (void)username;
+    (void)base_path;
+    return send_response(connection, 200, sv("TODO: Get\n"), false);
+}