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/if/if.wren

@@ -8,7 +8,3 @@ if (true) { IO.print("block") } // expect: block
 // Assignment in if condition.
 var a = false
 if (a = true) IO.print(a) // expect: true
-
-// Newline after "if".
-if
-(true) IO.print("good") // expect: good