fix(env): improve env detection compatibility for Node

[liamjones]

The React Native test environment is Node but sets up a window global: https://github.com/facebook/react-native/blob/7a85b911254dbae9fac4c6d8a862a20c33e0b36a/packages/react-native/Libraries/Core/setUpGlobals.js#L18

Reintroduce the navigator.userAgent check as a fallback for detecting a Node environment

[liamjones]

Without this tests in an RN environment fall over on this line:

zrender/src/core/env.ts

Line 90 in ae8cfaa

const style = document.documentElement.style;

As there's a window but no document

[plainheart]

Thanks for your contribution!