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/return/after_else.wren

@@ -1,3 +1,3 @@
-IO.print(fn {
+IO.print(new Fn {
   if (false) "no" else return "ok"
 }.call) // expect: ok

test/return/after_if.wren

@@ -1,3 +1,3 @@
-IO.print(fn {
+IO.print(new Fn {
   if (true) return "ok"
 }.call) // expect: ok

test/return/after_while.wren

@@ -1,3 +1,3 @@
-IO.print(fn {
+IO.print(new Fn {
   while (true) return "ok"
 }.call) // expect: ok

test/return/in_function.wren

@@ -1,4 +1,4 @@
-var f = fn {
+var f = new Fn {
   return "ok"
   IO.print("bad")
 }

test/return/return_null_if_brace.wren

@@ -1,4 +1,4 @@
-var f = fn {
+var f = new Fn {
   if (true) { return }
   IO.print("bad")
 }

test/return/return_null_if_newline.wren

@@ -1,4 +1,4 @@
-var f = fn {
+var f = new Fn {
   return
   IO.print("bad")
 }