Using ArcGIS API for JavaScript modules in a web worker causes "npm run build" to fail

[gavinr]

Vite has a nice feature where you can add ?worker to an import and it will create a web worker. (see https://vitejs.dev/guide/features.html#web-workers)

When using a module from the ArcGIS API for JavaScript in the web worker, "npm run dev" works fine but "npm run build" (which, I think uses Rollup), fails with the error:

To preserve the export signature of the entry module "src\testWorker.ts", an empty facade chunk was created. This often happens when creating a bundle for a web app where chunks are placed in script tags and exports are ignored. In this case it is recommended to set "preserveEntrySignatures: false" to avoid this and reduce the number of chunks. Otherwise if this is intentional, set "preserveEntrySignatures: 'strict'" explicitly to silence this warning.
✓ 10 modules transformed.
[vite:worker] Invalid value "iife" for option "output.format" - UMD and IIFE output formats are not supported for code-splitting builds.
file: C:/.../svelte-vite-web-workers-issue/src/testWorker.ts?worker
error during build:
Error: Invalid value "iife" for option "output.format" - UMD and IIFE output formats are not supported for code-splitting builds.

Replication Steps:

1. Clone this repo: https://github.com/gavinr/svelte-vite-web-workers-issue
2. npm install
3. npm run dev - notice it works fine
4. npm run build - build fails, with the above error (expected: build should work)