feat: validate argument type in Fn.new and add runtime error test

Add type checking to the `fn_new` native function to ensure the argument
passed to `Fn.new` is either a function or closure. Previously, passing
an invalid type like an integer would silently succeed. Now it returns
a clear runtime error message: "Argument must be a function."

Also remove two stale TODO comments about newline handling in the
compiler that were no longer relevant to current parsing logic.

test/function/new_with_wrong_argument_type.wren

@@ -0,0 +1 @@
+new Fn(3) // expect runtime error: Argument must be a function.