Try moving keytar to externals

This attempts to fix an extension compile error introduced with 3d933fb742

The issue is that keytar loads a `.node` file, which webpack is not able to understand

extensions/vscode-account/extension.webpack.config.js

@@ -16,5 +16,8 @@ module.exports = withDefaults({
 	},
 	entry: {
 		extension: './src/extension.ts',
-	}
+	},
+	externals: {
+		'keytar': 'commonjs keytar',
+	},
 });