Reorganize trailing comma code a bit.

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

test/language/list/duplicate_comma.wren

@@ -0,0 +1 @@
+[1,,2] // expect error

test/language/list/duplicate_trailing_comma.wren

@@ -0,0 +1 @@
+[1,,] // expect error

test/language/list/empty_list_with_comma.wren

@@ -0,0 +1 @@
+[,] // expect error

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

test/language/list/trailing_comma.wren

@@ -1,12 +1,4 @@
-var list = [
-  "a",
-  "b",
-]
+var list = ["a", "b",]
 
 IO.print(list[0]) // expect: a
 IO.print(list[1]) // expect: b
-
-// Invalid syntax
-IO.print(new Fiber { Meta.eval("[,]")    }.try()) // expect: Could not compile source code.
-IO.print(new Fiber { Meta.eval("[1,,]")  }.try()) // expect: Could not compile source code.
-IO.print(new Fiber { Meta.eval("[1,,2]") }.try()) // expect: Could not compile source code.