Hook beforeunload to cancel existing requests (#51)

We've been seeing passkeys on desktop Safari getting progressively less
reliable, as noted in [this
bug](https://bugs.webkit.org/show_bug.cgi?id=273712). My hunch is that
there's some sort of browser-wide state getting thrown off if requests
(in practice, only conditional) hang around.

To be clear, I'm not sure if this will actually work. I haven't
witnessed it causing any issues, and it seemed to minorly improve the
situation (but the system in question is completely opaque to me, so
it's just as likely coincidental) - after testing locally, Safari seemed
to be not _completely_ stuck after a couple cycles through. Other than a
handful of additional bytes, I don't see much of a downside here.

Author: Firehed

src/SDK.ts

@@ -59,6 +59,7 @@ class SDK {
   constructor(publicKey: string, host: string = 'https://api.snapauth.app') {
     this.apiKey = publicKey
     this.host = host
+    window.addEventListener('beforeunload', this.cancelExistingRequests)
   }
 
   get isWebAuthnAvailable() {