Since the `Visual Studio 18 2026` generator is new, we can switch
to the `.slnx` file format without changing behavior for users of
VS 2022 and older.
Fixes: #25887
Add a `VS_STARTUP_PROJECT` directory property to specify the project
that should be placed first in the `.sln` file so that it will be
selected as the default startup project.
Co-Author: Taylor Braun-Jones <taylor.braunjones@avigilon.com>
