[http-user-agent] Avoid using "new Function" to check browser environment

[emmenko]

Description

Using new Function is considered harmful and is blocked by CSP.

Since we are using it to check if the code runs on nodejs or browser, can find a better solution, thus allowing CSP to be happy about it.

[emmenko]

@tdeekens do you have a suggestion how we can easily check if the env is nodejs or browser?

[tdeekens]

I used something on DOMPurify which I managed to get by @cure53. Let me check.

[tdeekens]

Seems like we went for if (!window || !window.document || window.document.nodeType !== 9) {...} to detect if we run in the browser.

[tdeekens]

Let me spin up a PR.