fix(build): bundle non-inlined workers with rollup

[modderme123]

Fixes #2493

[antfu]

Tested with https://github.com/wheatjs/vueuse-playground by @wheatjs (using Monaco), it was not working properly on FireFox and with this PR now it does.

[Mesteery]

I changed this from es to iife because Firefox was running into trouble with default exports still being exported (something like unexpected top level export), so this was how I got rid of those imports and exports. I believe this module is standalone because you can't import code from a worker bundle so afaik this is not a breaking change, although it may make bundles a few bytes larger with the iife (Immediately Invoked Function Expression, which basically wraps the bundle with (()=>{code})() so that variables don't conflict with other bundles).

This is definitely a breaking change. The proof? Me.

I have to use an ES import in my worker, and since this commit it's impossible for me to do that...