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>

src/modules/cmdpal/extensionsdk/nuget/BuildSDKHelper.ps1

@@ -56,7 +56,7 @@ if ($IsAzurePipelineBuild) {
 }
 
 if (($BuildStep -ieq "all") -Or ($BuildStep -ieq "build")) {
-  & $nugetPath restore (Join-Path $PSScriptRoot "..\..\..\..\..\PowerToys.sln")
+  & $nugetPath restore (Join-Path $PSScriptRoot "..\..\..\..\..\PowerToys.slnx")
 
   Try {
     foreach ($config in $Configuration.Split(",")) {