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/is/is.wren

@@ -30,3 +30,7 @@ IO.print(123 is Class) // expect: false
 
 // TODO: Non-class on RHS.
 // TODO: Precedence and associativity.
+
+// Ignore newline after "is".
+IO.print(123 is
+  Num) // expect: true