Merge extensions into the main code base

[foxcpp]

Comments in #260 outline a number of issues caused by an attempt to keep extensions independent of the main go-imap code. This includes two extensions being unable to extend the same command.
I propose to ditch the idea of external extensions or at least seriously reconsider how it works for go-imap v2.

There is number of important IMAP extensions, notably IDLE, UIDPLUS and CONDSTORE. I think they should be supported by go-imap itself. This will simply make implementing its support on the server and client sides simpler.

Subtasks

• Revise the command parser in accordance with RFC 4466.
• Define Server-Backend interface suitable for use with multiple extensions at the same time.
• Define Client interface suitable for use with multiple extensions at the same time (e.g. UIDPLUS + CONDSTORE).
• Merge certain (probably all) extensions into the main code base.
  or
• Define new interfaces for use by external extensions.

Extensions to consider for merge

• IDLE
  RFC 2177, Implementation
  Rationale: Fundamental for useful clients and servers implementations.

• ENABLE
  RFC 53161, Implementation
  Rationale: Heavily used by other extensions

• CHILDREN
  RFC 3348, Implementation
  Rationale: Too small to be kept separate.

• SPECIAL-USE
  RFC 6154, Implementation
  Rationale: Too small to be kept separate.

• UNSELECT
  RFC 3691, Implementation
  Rationale: Too small to be kept separate.

... pretty much every small extension, currently maintained in a separate repository, see https://github.com/emersion/go-imap#extensions ...

References

• Collected Extensions to IMAP4 ABNF
  RFC 4466

[emersion]

Note that this isn't necessarily a breaking change, we can add new symbols as long as we don't break the current API.

[arnisoph]

👍 I would welcome having (all) extensions in one repo.

[emersion]

Closing because v2 does this from the start.