Reorganize trailing comma code a bit.

- Split out syntax error tests.
- Avoid redundant handling of empty literals.

test/language/list/newlines.wren

@@ -8,3 +8,17 @@ var list = [
 
 IO.print(list[0]) // expect: a
 IO.print(list[1]) // expect: b
+
+// Newline after trailing comma.
+list = ["c",
+
+]
+
+IO.print(list[0]) // expect: c
+
+// Newline in empty list.
+list = [
+
+]
+
+IO.print(list.count) // expect: 0