fix: avoid using window in module exports

[louis-bompart]

CDX-1593 🚀

Proposed changes:

dist/browser.mjs is the exported file in module.

The module can be imported by both NodeJS & browsers alike nowadays and as a library.

For example, we recently encountered an issue consuming the library after migrating Headless to an ESM-first package, as we export both ESM for NodeJS and browsers.

To solve that non-breakingly, I propose using the isomorphic way to access the global object instead of runtime specific, such as window or process.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis

From our experience in Headless, this is 'good enough', even if other places of the code uses window

How to test

Checklist:

• todo Unit tests and/or functional tests are written and cover the proposed changes ❗
• Unusure about how to tests, 🤔
• The proposed change can't affect any current customer implementation that could lead to events being rejected from our APIs or events being miscategorized by UA.
• Potentially no: If the library supports IE11, this change is not IE11 compatible, meaning events wouldn't be sent.