Add the silverstripe-extension to the devcontainer

.devcontainer/devcontainer.json

@@ -4,6 +4,7 @@
     "service": "silverstripe",
     "workspaceFolder": "/vscode/src/mantra",
     "extensions": [
+        "adrianhumphreys.silverstripe",
 		"dbaeumer.vscode-eslint",
         "felixfbecker.php-pack",
         "ms-vscode.vscode-typescript-tslint-plugin",
@@ -13,4 +14,4 @@
         "php.executablePath": null
     },
     "postCreateCommand": "echo '* text=auto eol=lf' > $(git rev-parse --git-dir)/info/attributes"
-}
+}