[btoa] Exporting the function in the legacy style

This definition was incorrectly declaring the function as a default
export, as opposed to a legacy-style on the exports directly.

Closes #28648

types/btoa/btoa-tests.ts

@@ -1,3 +1,3 @@
-import btoa from 'btoa';
+import btoa = require('btoa');
 
 btoa('foo');

types/btoa/index.d.ts

@@ -1,7 +1,10 @@
 // Type definitions for btoa 1.2
 // Project: https://git.coolaj86.com/coolaj86/btoa.js
 // Definitions by: John Wright <https://github.com/johngeorgewright>
+//                 Alex Brick <https://github.com/bricka>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.8
 
-export default function(str: string): string;
+declare function btoa(str: string): string;
+
+export = btoa;