refactor(packages/codegen): add `writeWithMapNamedPrivate` (#25981)

Continuation of #25980.

Add a specialized function `writeWithMapNamedPrivate` for writing a `PrivateIdentifier`. It feeds `hashLength: 1` to `markMapNamed`, which allows removing polymoprhic `node.type` access from `markMapNamed`.

The code being printed in paths that lead to `markMapNamed` is always the same as `node.name`, so just pass it in to `markMapNamed`, which removes a polymorphic `node.name` read there.

Additionally, `writeWithMapNamedNoLast` now only receives `Identifier` nodes, so becomes monomorphic.