feat: detect XCHATMAIL capability and expose it as is_chatmail config

src/config.rs

@@ -254,6 +254,9 @@ pub enum Config {
     /// True if account is configured.
     Configured,
 
+    /// True if account is a chatmail account.
+    IsChatmail,
+
     /// All secondary self addresses separated by spaces
     /// (`addr1@example.org addr2@example.org addr3@example.org`)
     SecondaryAddrs,