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