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/fiber/closure.wren

@@ -3,16 +3,16 @@ var closure
 
 {
   var a = "before"
-  fiber = Fiber.create(fn {
+  fiber = Fiber.create {
     IO.print(a)
     Fiber.yield
     a = "after"
     Fiber.yield
     IO.print(a)
     a = "final"
-  })
+  }
 
-  closure = fn {
+  closure = new Fn {
     IO.print(a)
   }
 }