clear terminal after switching to alternate screen

when using helix over mosh, the screen doesn't get cleared and
characters get left all over the place until they are overwritten. with
this change, the screen gets properly cleared as soon as helix starts

helix-term/src/application.rs

@@ -683,6 +683,7 @@ impl Application {
         terminal::enable_raw_mode()?;
         let mut stdout = stdout();
         execute!(stdout, terminal::EnterAlternateScreen)?;
+        execute!(stdout, terminal::Clear(terminal::ClearType::All))?;
         if self.config.load().editor.mouse {
             execute!(stdout, EnableMouseCapture)?;
         }