ESM: Added remaining Base64 ops and created a Base64 library. Added the prefer-const eslint rule.

src/web/LoaderWorker.js

@@ -25,7 +25,7 @@ self.addEventListener("message", function(e) {
  */
 self.loadFile = function(file) {
     const reader = new FileReader();
-    let data = new Uint8Array(file.size);
+    const data = new Uint8Array(file.size);
     let offset = 0;
     const CHUNK_SIZE = 10485760; // 10MiB