Move newline handling down into parser.

This lets us make it a grammar feature and not a lexer trick.
Newlines are explicitly ignored in some parts of the grammar. This
lets, for example, things like "|" ignore the newline when used as
an operator, but not when used as a parameter delimiter in a block
argument.

test/list/newline_before_comma.wren

@@ -0,0 +1,2 @@
+var list = ["a"
+, "b"] // expect error

test/list/newlines.wren

@@ -1,14 +1,10 @@
-// Allow newlines in most places.
+// Allow after '[' and ',', and before ']'.
 var list = [
 
-"a"
-, "b",
-
-"c", "d"
+"a",
+"b"
 
 ]
 
 IO.print(list[0]) // expect: a
 IO.print(list[1]) // expect: b
-IO.print(list[2]) // expect: c
-IO.print(list[3]) // expect: d