go/ast: add Preorder go1.23 iterator

go/ast: add Preorder go1.23 iterator

This CL adds a new function Preorder that makes it
easier to iterate over the nodes of a syntax tree.
In particular, break, continue, and return retain
their usual continuations.

Fixes #66339

Change-Id: I438b3c23780c91ed589871ad3b8822d54e8fabc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/570680
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>