Reorganize trailing comma code a bit.

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

test/language/map/newlines.wren

@@ -0,0 +1,27 @@
+// Allow after '{', ':', and ',', and before ']'.
+var map = {
+
+"a":
+
+"a value",
+
+"b": "b value"
+
+}
+
+IO.print(map["a"]) // expect: a value
+IO.print(map["b"]) // expect: b value
+
+// Newline after trailing comma.
+map = {"c": "c value",
+
+}
+
+IO.print(map["c"]) // expect: c value
+
+// Newline in empty map.
+map = {
+
+}
+
+IO.print(map.count) // expect: 0