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)

test/closure/close_over_function_parameter.wren

@@ -1,7 +1,7 @@
 var f = null
 
-fn(param) {
-  f = fn {
+new Fn {|param|
+  f = new Fn {
     IO.print(param)
   }
 }.call("param")

test/closure/close_over_method_parameter.wren

@@ -2,7 +2,7 @@ var f = null
 
 class Foo {
   method(param) {
-    f = fn {
+    f = new Fn {
       IO.print(param)
     }
   }

test/closure/closed_closure_in_function.wren

@@ -2,7 +2,7 @@ var f = null
 
 {
   var local = "local"
-  f = fn {
+  f = new Fn {
     IO.print(local)
   }
 }

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)

test/closure/open_closure_in_function.wren

@@ -1,6 +1,6 @@
 {
   var local = "local"
-  fn {
+  new Fn {
     IO.print(local) // expect: local
   }.call
 }

test/closure/reference_closure_multiple_times.wren

@@ -2,7 +2,7 @@ var f = null
 
 {
   var a = "a"
-  f = fn {
+  f = new Fn {
     IO.print(a)
     IO.print(a)
   }

test/closure/reuse_closure_slot.wren

@@ -3,7 +3,7 @@
 
   {
     var a = "a"
-    f = fn IO.print(a)
+    f = new Fn { IO.print(a) }
   }
 
   {

test/closure/use_global_in_function.wren

@@ -1,6 +1,6 @@
 var global = "global"
 // TODO: Forward reference to global declared after use.
 
-fn {
+new Fn {
   IO.print(global) // expect: global
 }.call