AuthenticationExtensions typedef name is confusing; also there are issues with its description

[equalsJeffH]

see 4.7. Authentication Extensions (typedef AuthenticationExtensions)

the name "AuthenticationExtensions" is confusing (to me at least) because "extensions" may be requested in both [[Create]](options) {#createCredential} and [[DiscoverFromExternalSource]](options) {#getAssertion} methods, and there are two extension subtypes: "authenticator extensions" and "client extensions".

Additionally, this same typedef is used to return extension results within the PublicKeyCredential object.

I suggest we rename this object "WebAuthenticationExtensionData". then having subtypes denoted as "authenticator extensions" and "client extensions" does not seem odd, because they are just abbreviated names for e.g. "authenticator web authentication extension" and "client web authentication extension".

I note that we do use the term "web authentication" in a few key manners, and it seems to me this ought to be one of them.

Additionally, I suggest we rename §8 WebAuthn Extensions to be "§8 Web Authentication Extensions".

I can address this as part of finishing PR #464.

[equalsJeffH]

the description issues are that the description needs to be updated to reflect that an WebAuthenticationExtensionData (nee AuthenticationExtension) instance is actually an ordered map.

[equalsJeffH]

actually, will do this in another PR, like a sweep-up-all-the-outstanding-extensions-stuff PR. #464 is complex enough.

[jcjones]

The under-specified behaviors are also an issue for #626

[jcjones]

We've agreed that the existing name matches up with the other Authenticator... names in our WebIDL, and we're going to stick with it.

The second half of this issue is addressed in #626, so closing this and we're keeping AuthenticationExtensions