feat: add async getters for session and user.

[lebrinkma]

What kind of change does this PR introduce?

adds async getters for session and user.

this provides a workaround for #143 and #23 without needing setTimeout.
this is an alternate solution to #147.

What is the current behavior?

After const supabase = createClient(supabaseUrl, supabaseKey)
supabase.auth.session() and supabase.auth.user() might return null,
if the session is expired and currently refreshing or if the session is recovering from a magic link.

What is the new behavior?

refreshedSession() resp. refreshedUser() wait for the pending recovery and
and return valid session resp. user data.

usage examples

const session = await supabase.auth.refreshedSession()
const user = await supabase.auth.refreshedUser()

Additional context

for supabase-js v2 (supabase/supabase-js#170)
user() and session() should IMHO be async functions.

[alaister]

Hey @lebrinkma,

Thanks for this - sorry I didn't see it sooner!

This is now being tracked in #285. Any comments you have are welcome there :)