Add ability to signal unknown credentials to the browser

[Stormheg]

Refs: #38

Tested with Chrome and Safari on macOS. Only Chrome appears to remove the Passkey if stored in the Chrome profile. When the iCloud Keychain is used, nothing appears to happen.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (51b2050) to head (d7106fc).
⚠️ Report is 25 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #93   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files          13       13           
  Lines         747      749    +2     
  Branches       65       65           
=======================================
+ Hits          747      749    +2     

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