build: move main and setup to SLNX (#43478)

Closes #37100

This does not migrate the rest of the solutions (why do we have so
many?)

Not migrated:

- TemplateCmdPalExtension.sln
- FancyZonesEditor.sln
- BugReportTool.sln
- CleanUp_tool.sln
- FancyZones_DrawLayoutTest.sln
- FancyZones_zonable_tester.sln
- FancyZone_HitTest.sln
- MonitorReportTool.sln
- PowerToyTemplate.sln
- StylesReportTool.sln

---------

Co-authored-by: vanzue <vanzue@outlook.com>

doc/devdocs/development/debugging.md

@@ -19,7 +19,7 @@ You can build the entire solution from the command line, which is sometimes fast
 2. Navigate to the repository root directory
 3. Run the following command(don't forget to set the correct platform):
    ```pwsh
-   msbuild -restore -p:RestorePackagesConfig=true -p:Platform=ARM64 -m PowerToys.sln /tl /p:NuGetInteractive="true"
+   msbuild -restore -p:RestorePackagesConfig=true -p:Platform=ARM64 -m PowerToys.slnx /tl /p:NuGetInteractive="true"
    ```
 4. This process should complete in approximately 13-14 minutes for a full build