Fix issue with window in worker

[marcofugaro]

Using window in a worker throws an error, let's use globalThis instead.

Its browser support seems aligned with our browser support specified here.

• globalThis browser support: https://caniuse.com/#feat=mdn-javascript_builtins_globalthis

• our supported browsers: https://browsersl.ist/?q=%3E1%25%2C+not+dead%2C+not+ie+11%2C+not+op_mini+all

Instead, if there is the need to support older browsers, we can easily transpile globalThis with babel by adding

useBuiltIns: 'usage',
corejs: { version: 3, proposals: true },

in the @babel/preset-env options and adding core-js as a dependency.