feat(jmap): client, additional providers

[toddheasley]

I wrote JMAP feature library initially (1) without a clear public interface and (2) with Fastmail hardcoded as the only service provider. PR addresses both as follows:

1. Adopt the same async/await public interface as IMAP and SMTP libraries, adding a JMAPClient configured with Server
2. Server swaps hardcoded token for Authorization enumeration that supports both user/password basic auth and bearer tokens

Additionally, PR removes subdirectories and flattens JMAP package sources files, accounting for extra renamed/moved file changes.

Close #78