Make COI-serviceworker optional?

[davcri]

This PR #89 fixed the web deploy.
However this configuration should not be the default and should be opt-in: if you are deploying a web game and you can set the appropriate HTTP headers it's better to configure them that way instead of relying on https://github.com/gzuidhof/coi-serviceworker

See also godotengine/godot-proposals#6616

[SecurityRisk]

I agree with this. Right now, if you run the project from within Godot, you'll get a 404 error in the browser console because this file wasn't added to the project. Making it optional or making it only get applied on GH Pages would be really useful

[davcri]

@SecurityRisk I'd like to know your opinion on #93. I left some comments there.

[davcri]

Relevant: https://godotengine.org/article/dev-snapshot-godot-4-3-dev-3/#single-threaded-web-exports

Update: https://godotengine.org/article/progress-report-web-export-in-4-3/#single-threaded-web-export