Recommend the CodeQL for VSCode extension

I've added a `.vscode/extensions.json` file that will automatically recommend the CodeQL for Visual Studio Code extension to anyone who opens the repo in VS Code (without the extension already installed).

Author: Dave Bartolomeo

.vscode/extensions.json

@@ -0,0 +1,12 @@
+{
+    // See https://go.microsoft.com/fwlink/?LinkId=827846 to learn about workspace recommendations.
+    // Extension identifier format: ${publisher}.${name}. Example: vscode.csharp
+
+    // List of extensions which should be recommended for users of this workspace.
+    "recommendations": [
+        "github.vscode-codeql"
+    ],
+    // List of extensions recommended by VS Code that should not be recommended for users of this workspace.
+    "unwantedRecommendations": [
+    ]
+}