* Step 1: Establish new `PYBIND11_SMART_HOLDER_ENABLED` macro, but only under include/pybind11/
At the stage `PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT` and `PYBIND11_SMART_HOLDER_ENABLED` are still equivalent.
* Systematically replace all `PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT` with `PYBIND11_SMART_HOLDER_ENABLED` under tests/ and ubench/
As at the previous stage, `PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT` and `PYBIND11_SMART_HOLDER_ENABLED` are still equivalent.
* Introduce `PYBIND11_SMART_HOLDER_DISABLE` option.
* `#ifdef` out entire `wrap()` function to avoid `unused-parameter` warning-as-error under macos-13
```
/Users/runner/work/pybind11/pybind11/tests/test_class_sh_trampoline_basic.cpp:67:23: error: unused parameter 'm' [-Werror,-Wunused-parameter]
void wrap(py::module_ m, const char *py_class_name) {
^
/Users/runner/work/pybind11/pybind11/tests/test_class_sh_trampoline_basic.cpp:67:38: error: unused parameter 'py_class_name' [-Werror,-Wunused-parameter]
void wrap(py::module_ m, const char *py_class_name) {
^
2 errors generated.
```
