feat(imap-types): add Capability::Imap4Rev2 variant for RFC 9051

[MarkAtwood]

Summary

RFC 9051 (IMAP4rev2) obsoletes RFC 3501. Servers and clients that implement it advertise IMAP4REV2 in their CAPABILITY response. Currently imap-types has Capability::Imap4Rev1 but no Capability::Imap4Rev2 variant, so any implementation that needs to advertise or parse this capability falls through to Capability::Other.

Requested change

Add a typed variant to the Capability enum:

pub enum Capability<'a> {
    Imap4Rev1,
    Imap4Rev2,  // RFC 9051
    // ... existing variants ...
}

With corresponding codec support (encode as IMAP4REV2, parse case-insensitively per RFC 9051 §9).

Context

RFC 9051 absorbs many extensions already modelled individually in imap-types (MOVE, NAMESPACE, ENABLE, LITERAL-, CONDSTORE, UTF8=ACCEPT, UIDPLUS, etc.). The individual extension variants are already there; the only missing piece is the top-level IMAP4REV2 capability string so that a server can advertise conformance to RFC 9051 as a whole.

References

• RFC 9051: https://www.rfc-editor.org/rfc/rfc9051
• Related extensions already in imap-types: NAMESPACE, MOVE, ENABLE, LITERAL-, UTF8, CONDSTORE

[duesee]

Hey @MarkAtwood! Thanks for the suggestion! I very much invite you to try doing a PR :-) The change seems straightforward and you basically identified what has to be done.