Export base64 utils

We use these from react-sdk (from crypto, when they were there,
but I just moved them and inadvertantly broke react-sdk).

src/matrix.ts

@@ -29,6 +29,7 @@ export * from "./http-api";
 export * from "./autodiscovery";
 export * from "./sync-accumulator";
 export * from "./errors";
+export * from "./base64";
 export * from "./models/beacon";
 export * from "./models/event";
 export * from "./models/room";