Replace .sln with .slnx; drop IDE-template scaffolding

The tracked GraphicalTools.sln carried stale ProjectReferences to
local Terminal.Gui sources, breaking CI's `dotnet test` (which was
implicitly resolving the solution). Switch to the modern .slnx format
and track it directly so the solution is reviewable and small.

- Replace GraphicalTools.sln with GraphicalTools.slnx (5-line XML;
  lists the 3 real projects, no dead Terminal.Gui ProjectReferences)
- Stop ignoring .slnx and .vscode/; track them
- Remove tools/initDevEnvironment.ps1 and tools/ide/ — no longer
  needed; .slnx is small enough to track and VS Code workspace
  files are now the supported way to share dev config
- Add .vscode/{launch,tasks,extensions}.json:
  * launch.json: single coreclr profile with a pickString input for
    selecting one of the OCGV / Show-ObjectTree sample commands
  * tasks.json: build (Invoke-Build), test (zero-arg dotnet test
    auto-resolves the .slnx), clean
  * extensions.json: recommends C# Dev Kit + PowerShell

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

.gitignore

@@ -4,11 +4,9 @@ bin/
 obj/
 publish/
 *.sln
-*.slnx
 
 # IDE support (keep locally, don't track)
 /.vs/
-/.vscode/
 *.user
 *.suo
 *.sln.DotSettings

GraphicalTools.sln

@@ -0,0 +1,5 @@
+<Solution>
+  <Project Path="src/Microsoft.PowerShell.ConsoleGuiTools/Microsoft.PowerShell.ConsoleGuiTools.csproj" />
+  <Project Path="src/Microsoft.PowerShell.OutGridView.Models/Microsoft.PowerShell.OutGridView.Models.csproj" />
+  <Project Path="test/Microsoft.PowerShell.ConsoleGuiTools.Tests/Microsoft.PowerShell.ConsoleGuiTools.Tests.csproj" />
+</Solution>