Formatter: Add trailing commas in lists

tests/formatting/lists_in.blp

@@ -0,0 +1,21 @@
+using Gtk 4.0;
+
+Box {
+  styles []
+}
+
+Box {
+  styles ["a"]
+}
+
+Box {
+  styles ["a",]
+}
+
+Box {
+  styles ["a", "b"]
+}
+
+Box {
+  styles ["a", "b",]
+}

tests/formatting/lists_out.blp

@@ -0,0 +1,31 @@
+using Gtk 4.0;
+
+Box {
+  styles []
+}
+
+Box {
+  styles [
+    "a",
+  ]
+}
+
+Box {
+  styles [
+    "a",
+  ]
+}
+
+Box {
+  styles [
+    "a",
+    "b",
+  ]
+}
+
+Box {
+  styles [
+    "a",
+    "b",
+  ]
+}

tests/formatting/out.blp

@@ -11,7 +11,7 @@ Overlay {
     notify::icon-name => $on_icon_name_changed(label) swapped;
 
     styles [
-      "destructive"
+      "destructive",
     ]
   }