Add AnyNodeRef.visit_preorder

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## Summary

This PR adds the `AnyNodeRef.visit_preorder` method. I'll need this method to mark all comments of a suppressed node's children as formatted (in debug builds). 

I'm not super happy with this because it now requires a double-dispatch where the `walk_*` methods call into `node.visit_preorder` and the `visit_preorder` then calls back into the visitor. Meaning,
the new implementation now probably results in way more function calls. The other downside is that `AnyNodeRef` now contains code that is difficult to auto-generate. This could be mitigated by extracting the `visit_preorder` method into its own `VisitPreorder` trait. 

Anyway, this approach solves the need and avoids duplicating the visiting code once more. 

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## Test Plan

`cargo test`

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