CI for smart holders as default

[rhaschke]

This adds another CI workflow enabling PYBIND11_USE_SMART_HOLDER_AS_DEFAULT

[rhaschke]

cmake implicitly sets CMAKE_CXX_FLAGS="/GR /EHsc" for MSVC. Manually setting CMAKE_CXX_FLAGS overrides this default and thus requires adding those flags explicitly.

[rwgk]

I really like that it's so easy, but I'm also really worried about introducing two copies of something so substantial. @henryiii, is there an obvious way to avoid the duplication?

Naive idea: have a small patch file somewhere else, and a bash script that applies that patch to generate this file, with a big comment at the top: "This is an auto-generated file, do not edit." And another comment with instructions how to run the bash script. — But I hope Henry knows of something more organized.

[rhaschke]

I tried to merge the new SH builds into the existing CI workflow before, but that ended up in a mess, because the workflow defines its jobs using matrix + includes. Doing all jobs with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT enabled required way more changes to the CI workflow. With the present approach, it should be easier to keep the CI and CI SH workflows in sync.

[rwgk]

The work here was included in #2963. Closing. Thanks Robert!