Standardize method names for clarity

[Firehed]

Related to #48 (and, to lesser degrees, #36 and #47) , keeping the method names more consistent would also be quite valuable.

Here's about where I'm at, modulo naming tweaks from #48:

function createCredential({ username: string }) // + displayName
function createInBackground({ username: string }) // ^

function auth({ id, username }) // or, not both
function autofill()

I somewhat like keeping the object arguments, since the structural typing of TS (and general vanilla JS semantics) should let you provide an already-hydrated user object in a lot of cases, assuming it has an id property (probably common) and/or username (realistically less common, although still works nice on many auth forms)