Conditional Mediation feature discovery should really return a promise

[nsatragno]

(filing a new issue for this as it's a small change and I don't want to pile more stuff on the conditional mediation issue / pr)

@emlun pointed out that since isUserVerifyingPlatformAuthenticatorAvailable() returns a promise, maybe isConditionalMediationAvailable() should return a promise as well. I said no because the browser could figure this out quickly without blocking, but after implementing, I realized I was wrong (-:. The details are that on Windows, we'll gate availability on dynamically determining if the webauthn DLL exposes required functions, which is slow.

Let's have isConditionalMediationAvailable return a Promise<bool> with the availability instead.