Use native AbortController when possible.

[Gozala]

This library uses browser field of package.json in order to avoid including a shim and uses own implementation otherwise. That however creates problems when used in Electron (main process) because there it loads a own implementation which native APIs like fetch reject with error like:

TypeError: Failed to execute 'fetch' on 'Window': member signal is not of type AbortSignal.

It would be nice if implementation still checked for presence of native AbortSignal just in case it exists.

[martinheidegger]

Just in case other people stumble over this, there is the package https://github.com/achingbrain/native-abort-controller that works around this particular issue.