chore: improve d.ts with ts support

index.d.ts

@@ -165,6 +165,29 @@ declare module 'egg' {
 
   export type LoggerLevel = 'DEBUG' | 'INFO' | 'WARN' | 'ERROR' | 'NONE';
 
+  /**
+   * egg app info
+   * @example
+   * ```js
+   * // config/config.default.ts
+   * import { EggAppInfo } from 'egg';
+   * 
+   * export default (appInfo: EggAppInfo) => {
+   *   return {
+   *     keys: appInfo.name + '123456',
+   *   };
+   * }
+   * ```
+   */
+  export interface EggAppInfo {
+    pkg: string; // package.json
+    name: string; // the application name from package.json
+    baseDir: string; // current directory of application
+    env: string; // equals to serverEnv
+    HOME: string; // home directory of the OS
+    root: string; // baseDir when local and unittest, HOME when other environment
+  }
+
   export interface EggAppConfig {
     workerStartTimeout: number;
     baseDir: string;