Fix squiggle-lang workers in hub

packages/squiggle-lang/package.json

@@ -13,7 +13,7 @@
     "build": "pnpm run build:peggy && pnpm run build:ts && pnpm run build:worker",
     "build:peggy": "peggy --cache --format es ./src/ast/peggyParser.peggy",
     "build:ts": "tsc -b",
-    "build:worker": "esbuild src/runners/worker.ts --bundle --minify --platform=browser --outfile=dist/runners/esbuild-worker.cjs",
+    "build:worker": "esbuild src/runners/worker.ts --bundle --minify --platform=browser --format=esm --outfile=dist/runners/esbuild-worker.js",
     "dev": "tsc -b -w",
     "clean": "rm -rf dist && rm -f src/ast/peggyParser.js && rm *.tsbuildinfo",
     "jest": "NODE_OPTIONS=--experimental-vm-modules jest",

packages/squiggle-lang/src/runners/WebWorkerRunner.ts

@@ -14,18 +14,11 @@ export class WebWorkerRunner extends AnyWorkerRunner {
      * If you need to touch this code, make sure to check carefully both the components (storybook) and the hub editor.
      * This configuration and the esbuild configuration in `package.json` are extremely fragile.
      *
-     * In particular:
-     * 1. ESM is problematic:`{ type: "module" }` (with `.mjs` or `.js`)will seemingly work, but cause lots of Turbopack CLI errors; Turbopack fails to analyze `(url, { type: "module" })` parameters correctly, as of Next.js 15.0.3.
-     * 2. `.js` extension will collide with the default `"type": "module"` in `package.json`, which will lead to Next.js warnings.
-     *
-     * So, instead, we bundle the worker to the basic IIFE; see `package.json` for details.
-     *
-     * This might cause issues in the future, if we decide to depend on some library that's ESM-only; I'm not sure how good esbuild is for bundling ESM dependencies.
-     *
-     * But, for now, it seems like it's working.
+     * Details here: https://github.com/quantified-uncertainty/squiggle/pull/3456
      */
-    const url = new URL("./esbuild-worker.cjs", import.meta.url);
-    this.worker = new Worker(url);
+    this.worker = new Worker(new URL("./esbuild-worker.js", import.meta.url), {
+      type: "module",
+    });
   }
 
   async getWorker(): Promise<Worker> {