Request mocking

- mock:
    regex: "/todos" # URL matcher; also accepts substring, glob, or domain
    method: GET
    responseGenerator: |
      const url = new URL(mock.request.url);

      const fakeTodo = {
        id: url.searchParams.get("id"),
        name: "My todo item",
        done: false,
        description: "An item that I need to do",
      };

      return new Response(JSON.stringify([fakeTodo]), {
        status: 200,
        headers: { "content-type": "application/json" },
      });

- mock:
    regex: "/api/flags"
    fetchOriginalResponse: true
    responseGenerator: |
      const originalJson = await mock.response.json();

      return new Response(
        JSON.stringify({
          ...originalJson,
          flags: {
            ...originalJson.flags,
            momentic_test: true,
          },
        }),
        {
          status: mock.response.status,
          headers: mock.response.headers,
        },
      );