Don't override global fetch & fix tests

[AntoineAtMistral]

Use globalThis.mistralFetch instead of globalThis.fetch to avoid overriding it.
Fix isNode logic.

Thanks @flakey5 for the contribution.

[sublimator]

Some comments:

window is also not defined in service workers, as used by Web Extensions, using manifest version 3

It seems the isNode flag is used to determine when to return the response.body directly from the _request method, vs implementing an async iterator.

In fact, in newer versions of node, with native fetch you need to do much the same, so isNode is a somewhat deceptive name here. It's more node-fetch specific if I'm not mistaken

In any case, this library should work in:

1. browser in a normal page
2. browser in a service worker
3. an older version of node, without native fetch
4. a newer version of node, with native fetch