[smart_holder] Configure only one job per OS kind to build and run tests with `-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT` (#5307)

* Configure only one job per OS kind to build and run tests with `-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT`

Remove ci_sh_def.yml

Piggy-backed: Add `smart_holder` to `branches` in emscripten.yaml

* Fix misunderstanding.

* Add `/GR /EHsc` for Windows.

* #error "Intentional Breakage" (to verify: PYBIND11_ACTUALLY_USING_SMART_HOLDER_AS_DEFAULT)

* Revert "#error "Intentional Breakage" (to verify: PYBIND11_ACTUALLY_USING_SMART_HOLDER_AS_DEFAULT)"

This reverts commit 4f6f5cb6e907ab2352733c4756bf68662e9c3f01.