Fix minified worker module

[edoardocavazza]

This PR attempts to fix minified code for the worker module.

Issue: #2748

The problem here is that the final worker code is transformed into an UMD module that defines the module variable.
The internal Lottie code runs in the worker, but it is loaded using toString on a function that is affected by minification. When the minifier run, it thinks module is defined and then remove any other check.
This PR resets the module variabile definition in the wrapper.

it ss a kind of workaround, but I think is safer than rewriting the whole Lottie module system 😅