expose more FS methods to ts

So I can use them in typescript project without syntax error.

.gitignore

@@ -2,3 +2,6 @@ node_modules
 dist
 /.nyc_output
 .DS_Store
+
+# ide
+.idea/

src/index.d.ts

@@ -2,6 +2,8 @@ export const FS: {
     writeFile: (fileName: string, binaryData: Uint8Array) => void,
     readFile: (fileName: string) => Uint8Array,
     unlink: (fileName: string) => void,
+    mkdir: (fileName: string) => void,
+    readdir: (fileName: string) => string[],
 }
 
 type FSMethodNames = { [K in keyof typeof FS]: (typeof FS)[K] extends (...args: any[]) => any ? K : never }[keyof typeof FS];