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/conditional/missing_else.wren

@@ -1 +1 @@
-true ? 1 : // expect error
+(true ? 1 :) // expect error

test/conditional/missing_then.wren

@@ -1 +1 @@
-true ? : 2 // expect error
+(true ? : 2) // expect error

test/conditional/newlines.wren

@@ -0,0 +1,7 @@
+// Newline after '?'.
+IO.print(true ?
+  "yes" : "no") // expect: yes
+
+// Newline after ':'.
+IO.print(false ? "yes" :
+  "no") // expect: no