feat(riff-raff.yaml): Introduce GuRoot

The `GuRoot` class should allow for easy adoption of `riff-raff.yaml` generation.

Going from:

```ts
// bin/cdk.ts
import { App } from "aws-cdk-lib";

const app = new App();

new MyStack(app, "some-id", {});
```

To:

```ts
// bin/cdk.ts
import { GuRoot } from "@guardian/cdk/lib/constructs/root";

const app = new GuRoot();

new MyStack(app, "some-id", {});
```

src/constructs/root.ts

@@ -0,0 +1,13 @@
+// Ideally this file would sit in `constructs/core`, however this creates violations of the `import/no-cycle` linting rule.
+import type { StageSynthesisOptions } from "aws-cdk-lib";
+import { App } from "aws-cdk-lib";
+import type { CloudAssembly } from "aws-cdk-lib/cx-api";
+import { RiffRaffYamlFile } from "../utils/riff-raff-yaml-file";
+
+// Deliberately not called `GuApp` as `app` is overloaded (as is `stack`, and `stage` 😅)
+export class GuRoot extends App {
+  override synth(options?: StageSynthesisOptions): CloudAssembly {
+    new RiffRaffYamlFile(this).synth();
+    return super.synth(options);
+  }
+}