feat: add devcontainer.json so users can open in codespaces (#469)

### Motivation and Context
Make it easier for newcomers to try out the sample apps and notebooks.

### Description
Add DevContainer configuration so users can get quickly get started
using VS Code DevContainers or GitHub Codespaces. This DevContainer
includes all tools and extensions necessary to begin testing sample apps
and notebooks (e.g., dotnet, node, az function core tools).

.devcontainer/devcontainer.json

@@ -0,0 +1,18 @@
+{
+	"image": "mcr.microsoft.com/devcontainers/universal:2",
+	"features": {
+		"ghcr.io/devcontainers/features/node:1": {},
+		"ghcr.io/devcontainers/features/dotnet:1": {},
+		"ghcr.io/jlaundry/devcontainer-features/azure-functions-core-tools:1": {}
+	},
+	"customizations": {
+		"vscode": {
+			"extensions": [
+				"ms-dotnettools.dotnet-interactive-vscode",
+				"ms-semantic-kernel.semantic-kernel",
+				"esbenp.prettier-vscode"
+			]
+		}
+	},
+	"postCreateCommand": "sudo chmod a+rwx /usr/share/dotnet" // avoids needing to run as 'sudo' when starting KernelHttpServer
+}