Fix environment detection in web worker

[andrewbranch]

Fixes #102

[andrewbranch]

@guybedford thanks! Is there a particular release cadence to npm for this library?

[guybedford]

@andrewbranch thinking about this more I'm not quite sure this is the right fix still actually - in a cloudflare worker, we'd hit the condition despite Buffer being defined.

So I'm tempted to say the check should actually be based on typeof Buffer !== 'undefined'.

Further, we should probably default to using typeof crypto !== 'undefined' && typeof crypto.subtle !== 'undefined' at this point in time as well now I think too?