pybind11

Small cleanup/refactoring in support of PR #5213 (#5251)

* Factor out detail/value_and_holder.h (from detail/type_caster_base.h)

This is in support of PR #5213:

* trampoline_self_life_support.h depends on value_and_holder.h

* type_caster_base.h depends on trampoline_self_life_support.h

* Fix a minor and inconsequential inconsistency in `copyable_holder_caster`: the correct `load_value()` return type is `void` (as defined in `type_caster_generic`)

For easy future reference, this is the long-standing inconsistency:

* https://github.com/pybind/pybind11/blob/dbf848aff7c37ef8798bc9459a86193e28b1032f/include/pybind11/detail/type_caster_base.h#L634

* https://github.com/pybind/pybind11/blob/dbf848aff7c37ef8798bc9459a86193e28b1032f/include/pybind11/cast.h#L797

Noticed in passing while working on PR #5213.

* Add `DANGER ZONE` comment in detail/init.h, similar to a comment added on the smart_holder branch (all the way back in 2021).