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/closure/nested_closure.wren

@@ -1,12 +1,12 @@
 var f = null
 
-fn {
+new Fn {
   var a = "a"
-  fn {
+  new Fn {
     var b = "b"
-    fn {
+    new Fn {
       var c = "c"
-      f = fn {
+      f = new Fn {
         IO.print(a)
         IO.print(b)
         IO.print(c)