Add devcontainer.json to support both .NET 8 and .NET 9 in GitHub Codespaces

Co-authored-by: eiriktsarpalis <[email protected]>

Author: Copilot

.devcontainer/devcontainer.json

@@ -0,0 +1,19 @@
+{
+	"name": "C# (.NET SDK)",
+	"image": "mcr.microsoft.com/devcontainers/dotnet:1-8.0-jammy",
+	"features": {
+		"ghcr.io/devcontainers/features/dotnet:2": {
+			"version": "9.0",
+			"additionalVersions": "8.0"
+		}
+	},
+	"customizations": {
+		"vscode": {
+			"extensions": [
+				"ms-dotnettools.csharp",
+				"ms-dotnettools.csdevkit"
+			]
+		}
+	},
+	"postCreateCommand": "dotnet --list-sdks"
+}