Make backup dir name config variable optional

dist/data/app-data.d.ts

@@ -1,4 +1,5 @@
 export declare const AppData: {
     readonly ConfigFileName: "bun-sqlite.config.ts";
     readonly MaxBackups: 10;
+    readonly DefaultBackupDirName: ".backups";
 };

dist/data/app-data.js

@@ -1,4 +1,5 @@
 export const AppData = {
     ConfigFileName: "bun-sqlite.config.ts",
     MaxBackups: 10,
+    DefaultBackupDirName: ".backups",
 };