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

## 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:

```rust
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