added alt/else, loops and other group shortcuts

src/diagram.typ

@@ -185,7 +185,7 @@
 
   // Compute groups spans (horizontal)
   for (i, elmt) in elmts.enumerate() {
-    if elmt.type == "grp" {
+    if elmt.type == "grp" or elmt.type == "alt" {
       let (min-i, max-i) = get-group-span(participants, elmt)
       elmts.at(i).insert("min-i", min-i)
       elmts.at(i).insert("max-i", max-i)