Refactor Megamix 1

[MasterKale]

This is a pretty extensive PR intended to make it easier to pass info from browser to server by exposing more of the data structures returned from the browser start methods to the complimentary verification methods in server. This should make it easier to track what values are being returned from these calls and enhance them with future return values as the spec evolves. It should also make it easier to test these libraries for conformance with the spec.

For more context on the original intent of this PR, see #20.

There was also some mixed use of "base64" encoding when, according to the spec, "base64url" encoding is required. This was due to me, in my naivety, conflating the two. It should now be easier to tell when these encoded values are being used versus the ArrayBuffers they represent.

For more detailed information on the changes in this PR, see below:

• [browser] (BREAKING) Refactor startAttestation() and startAssertion() to return more of the output from the navigator.credentials calls
• [browser] Replace base64-js dependency with internal functionality
• [browser, server] Standardize on use of Base64URL encoding when converting to and from JSON
• [server] (BREAKING) Remove references to "base64" from generateAttestationOptions() and generateAssertionOptions() by renaming the excludedBase64CredentialIDs and allowedBase64CredentialIDs to excludedCredentialIDs and allowedCredentialIDs respectively
• [typescript-types] (BREAKING) Migrate some non-shared typings into server

[MasterKale]

These changes have been released as v0.5.0