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/assign_to_closure.wren

@@ -3,13 +3,13 @@ var g = null
 
 {
   var local = "local"
-  f = fn {
+  f = new Fn {
     IO.print(local)
     local = "after f"
     IO.print(local)
   }
 
-  g = fn {
+  g = new Fn {
     IO.print(local)
     local = "after g"
     IO.print(local)