WebAuthnIDL: Default userVerification should be "preferred" per spec

[AlfioEmanueleFresta]

The from_json() parsing defaults userVerification to Discouraged when authenticatorSelection is not present:

libwebauthn/libwebauthn/src/ops/webauthn/make_credential.rs

Lines 364 to 368 in d97c80d

	impl FromInnerModel<PublicKeyCredentialCreationOptionsJSON, MakeCredentialRequestParsingError>
	for MakeCredentialRequest
	{
	fn from_inner_model(
	rpid: &RelyingPartyId,

Per the WebAuthn spec, the default should be "preferred":
https://www.w3.org/TR/webauthn-3/#dom-authenticatorselectioncriteria-userverification

userVerification, of type DOMString, defaulting to "preferred"