Reorganize trailing comma code a bit.

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

test/language/map/duplicate_comma.wren

@@ -0,0 +1 @@
+{1:1,,2:2} // expect error

test/language/map/duplicate_trailing_comma.wren

@@ -0,0 +1 @@
+{1:1,,} // expect error

test/language/map/empty_map_with_comma.wren

@@ -0,0 +1 @@
+{,} // expect error

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

test/language/map/trailing_comma.wren

@@ -5,9 +5,3 @@ var map = {
 
 IO.print(map["a"]) // expect: 1
 IO.print(map["b"]) // expect: 2
-
-// Invalid syntax
-// Parentheses are necessary to have these interpret as maps and not as blocks.
-IO.print(new Fiber { Meta.eval("({,})")        }.try()) // expect: Could not compile source code.
-IO.print(new Fiber { Meta.eval("({1:1,,})")    }.try()) // expect: Could not compile source code.
-IO.print(new Fiber { Meta.eval("({1:1,,2:2})") }.try()) // expect: Could not compile source code.