Add initial support for Visual Studio 2026

[jamesobutler]

See https://devblogs.microsoft.com/visualstudio/visual-studio-2026-insiders-is-here/

https://devblogs.microsoft.com/cppblog/c-language-updates-in-msvc-build-tools-v14-50/

Visual Studio 2026 in GitHub hosted runners progress at https://github.com/actions/runner-images/issues/13035

[jcfr]

To help understand the failures:

• ci: Run tests with additional verbosity #18

[jamesobutler]

Testing: https://github.com/scikit-build/cmake-FindVcvars/actions/runs/17952851801/job/51056618203?pr=17

1: CMake Error at D:/a/cmake-FindVcvars/cmake-FindVcvars/src/FindVcvars.cmake:391 (cmake_host_system_information):
1: cmake_host_system_information does not recognize VS_18_DIR

VS_18_DIR is available in CMake as of this commit. It will be available in CMake 4.2.0 estimated Oct 1st release (https://gitlab.kitware.com/cmake/cmake/-/milestones/177#tab-issues). GitHub Action jwlawson/actions-setup-cmake doesn't appear to support pulling nightly builds of CMake, so will need to wait for the 4.2.0 release.