Allow importing prebuilt bundle in ES6 ESM

[DarkNami]

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Feature Description

I want to import (use) Fullcalendar on browser directly (without bundler) but it does not work.
Because variable 'Fullcalendar' is not declared in 'window' object, I can not access 'Fullcalendar' object from main scope.

I want to create a PR but it is too hard for me. So I feedback a little suggestion:

Declare <script> version 'Fullcalendar' in 'window' object:

Developers can import it and use it on browser directly:

Thank you.

Ref:

• Import a module for its side effects only