Add sendSignal() helper

[MasterKale]

This PR adds a new sendSignal() method to @simplewebauthn/browser to simplify use of the WebAuthn Signal APIs:

import { sendSignal } from '@simplewebauthn/browser';

await sendSignal({
  signalName: 'unknownCredential',
  rpID: 'example.com',
  credentialID: 'aPH2yv3TIBVZbj2wJ30Kmw',
});

await sendSignal({
  signalName: 'allAcceptedCredentials',
  rpID: 'example.com',
  userID: 'Lrk2ki-arSSFcm6PPlMKEQ',
  allAcceptedCredentialIDs: [
    'd77Hxqvltn66l7hegq_E3Q',
    'nz_ajzNrMq3KfPrkOt3lHQ',
  ]
});

await sendSignal({
  signalName: 'currentUserDetails',
  rpID: 'example.com',
  userID: 'Lrk2ki-arSSFcm6PPlMKEQ',
  userName: 'newUsername@example.com',
  userDisplayName: 'NewUsername (Prod)',
});

Fixes #718