TS: add typings for 'src/utils/base64.js' (#372)

Author: IvanGoncharov

src/utils/base64.d.ts

@@ -0,0 +1,5 @@
+export declare type Base64String = string;
+
+export declare function base64(input: string): Base64String;
+
+export declare function unbase64(input: Base64String): string;