Holodeck: Change ResponseGenerator to return JSONValue

[lucasxzh]

Holodeck should be able to support mocking of servers that aren't following JSON:API spec.

async function registerProjectMessages(ctx: TestContext, id: string) {
  // @ts-expect-error - Holodeck types don't support empty arrays yet? but this should be valid
  await GET(ctx, `projects/${id}/messages`, () => [], { RECORD: true });
}

[runspired]

needs prettier run but otherwise seems good