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/while/newline_after_while.wren

@@ -0,0 +1,2 @@
+while // expect error
+(true) IO.print("bad")

test/while/syntax.wren

@@ -14,11 +14,3 @@ while (a < 3) {
 // expect: 0
 // expect: 1
 // expect: 2
-
-// Newline after "while".
-var d = 0
-while
-(d < 3) IO.print(d = d + 1)
-// expect: 1
-// expect: 2
-// expect: 3