Plumb OSC 777 to toast notifications

```
echo -e "\033]777;notify;This is the notification title;This is the notification text\a"
```

Now pops up a notification in a similar manner to OSC 9, except
that this form allows setting both the title and the body separately.

refs: https://github.com/wez/wezterm/issues/489

term/src/terminalstate.rs

@@ -3192,6 +3192,27 @@ impl<'a> Performer<'a> {
                     log::info!("Application sends SystemNotification: {}", message);
                 }
             }
+            OperatingSystemCommand::RxvtExtension(params) => {
+                if let Some("notify") = params.get(0).map(String::as_str) {
+                    let title = params.get(1);
+                    let body = params.get(2);
+                    let (title, body) = match (title.cloned(), body.cloned()) {
+                        (Some(title), None) => (None, title),
+                        (Some(title), Some(body)) => (Some(title), body),
+                        _ => {
+                            log::error!("malformed rxvt notify escape: {:?}", params);
+                            return;
+                        }
+                    };
+                    if let Some(handler) = self.notification_handler.as_mut() {
+                        handler.show_notification(ToastNotification {
+                            title,
+                            body,
+                            focus: true,
+                        });
+                    }
+                }
+            }
             OperatingSystemCommand::CurrentWorkingDirectory(url) => {
                 self.current_dir = Url::parse(&url).ok();
             }

termwiz/src/escape/osc.rs

@@ -44,6 +44,7 @@ pub enum OperatingSystemCommand {
     ResetDynamicColor(DynamicColorNumber),
     CurrentWorkingDirectory(String),
     ResetColors(Vec<u8>),
+    RxvtExtension(Vec<String>),
 
     Unspecified(Vec<Vec<u8>>),
 }
@@ -300,6 +301,13 @@ impl OperatingSystemCommand {
             ITermProprietary => {
                 self::ITermProprietary::parse(osc).map(OperatingSystemCommand::ITermProprietary)
             }
+            RxvtProprietary => {
+                let mut vec = vec![];
+                for slice in osc.iter().skip(1) {
+                    vec.push(String::from_utf8_lossy(slice).to_string());
+                }
+                Ok(OperatingSystemCommand::RxvtExtension(vec))
+            }
             FinalTermSemanticPrompt => self::FinalTermSemanticPrompt::parse(osc)
                 .map(OperatingSystemCommand::FinalTermSemanticPrompt),
             ChangeColorNumber => Self::parse_change_color_number(osc),
@@ -473,6 +481,7 @@ impl Display for OperatingSystemCommand {
             SetIconNameSun(title) => single_string!(SetIconNameSun, title),
             SetHyperlink(Some(link)) => link.fmt(f)?,
             SetHyperlink(None) => write!(f, "8;;")?,
+            RxvtExtension(params) => write!(f, "777;{}", params.join(";"))?,
             Unspecified(v) => {
                 for (idx, item) in v.iter().enumerate() {
                     if idx > 0 {
@@ -1466,6 +1475,21 @@ mod test {
         );
     }
 
+    #[test]
+    fn rxvt() {
+        assert_eq!(
+            parse(
+                &["777", "notify", "alert user", "the tea is ready"],
+                "\x1b]777;notify;alert user;the tea is ready\x1b\\"
+            ),
+            OperatingSystemCommand::RxvtExtension(vec![
+                "notify".into(),
+                "alert user".into(),
+                "the tea is ready".into()
+            ]),
+        )
+    }
+
     #[test]
     fn iterm() {
         assert_eq!(