Convert .sln to .slnx format #649

Job	Run time
build-devcontainer	1m 0s
build (Debug, windows-latest)	16m 14s
build (Debug, macOS-latest)	15m 1s
build (Debug, ubuntu-latest)	12m 14s
build (Release, ubuntu-latest)	12m 3s
build (Release, windows-latest)	16m 41s
build (Release, macOS-latest)	11m 31s
	1h 24m 44s

Provide feedback