Implement IMAP REPLACE extension, RFC 8508.

REPLACE can be used to update draft messages as you are editing. Instead of
requiring an APPEND and STORE of \Deleted and EXPUNGE. REPLACE works
atomically.

It has a syntax similar to APPEND, just allows you to specify the message to
replace that's in the currently selected mailbox. The regular REPLACE-command
works on a message sequence number, the UID REPLACE commands on a uid. The
destination mailbox, of the updated message, can be different. For example to
move a draft message from the Drafts folder to the Sent folder.

We have to do quite some bookkeeping, e.g. for updating (message) counts for
the mailbox, checking quota, un/retraining the junk filter. During a
synchronizing literal, we check the parameters early and reject if the replace
would fail (eg over quota, bad destination mailbox).

imapclient/protocol.go

@@ -39,6 +39,7 @@ const (
 	CapCompressDeflate  Capability = "COMPRESS=DEFLATE"   // ../rfc/4978:65
 	CapListMetadata     Capability = "LIST-METADTA"       // ../rfc/9590:73
 	CapMultiAppend      Capability = "MULTIAPPEND"        // ../rfc/3502:33
+	CapReplace          Capability = "REPLACE"            // ../rfc/8508:155
 )
 
 // Status is the tagged final result of a command.