VS: Add SLNX default startup project

At the time of commit e6aa7742b0 (VS: Generate .slnx files for VS 2026,
2025-09-17, v4.2.0-rc1~151^2), the VS 18 Insiders edition did not offer
support for specifying a default startup project in `.slnx` files.
This has since been added, so use it.

Fixes: #27387