Standardize parameter names for clarity

[Firehed]

The initial WebAuthn spec wasn't entirely clear about the semantics of name and displayName; consequently, there were some inconsistencies in the SDK about what to use and where. While the spec hasn't changed materially, some WWDC sessions in particular have started to add clarity here: user.name should be what the user would type in a username field during a non-passkey flow (currently called handle), and user.displayName should be a friendly/given name.

Let's set the names and APIs up for success here.

UserRegistrationInfo should combine name and handle into a single value, probably username, and keep the client-only management of this data. id can stay as-is, reserved for future use (though I expect it'll never be used in registration, and only used in signal APIs once they're supported). displayName is also probably OK as-is.

UserAuthenticationInfo should rename handle to whatever UserRegistrationInfo lands on (e.g. username).

I could see a simplification of the APIs to avoid fussing with objects, though it tends to provide less validation. Roughly:

interface SDK {
  async startRegister(username: string, displayName: string = undefined);
  async startAuthByUsername(username: string);
  async startAuthById(id: string);
}

Update: the above has been split into #49.