feat: migrate all function literals from fn syntax to new Fn block syntax

Replace all occurrences of the old `fn` function literal syntax with the new `new Fn {|params| body}` block syntax across benchmark, test, and source files. Update the Wren compiler to handle the pipe operator as a block argument delimiter by removing `TOKEN_PIPE` from the newline-swallowing token list and adding explicit newline matching in `infixOp`. Adjust test expectations in `conditional/precedence.wren` to remove `fn`-based expressions and update `test/assignment/local.wren` to use the new syntax.

test/super/closure.wren

@@ -4,9 +4,7 @@ class Base {
 
 class Derived is Base {
   getClosure {
-    return fn {
-      return super.toString
-    }
+    return new Fn { super.toString }
   }
 
   toString { return "Derived" }

test/super/super_in_top_level_function.wren

@@ -1,3 +1,3 @@
-fn {
+new Fn {
   super.foo // expect error
 }