[dt serve] Support running in machine mode and when `--run-app` is true

When the server is started with `--machine`, it uses machine-readable JSON as the output. Using`--run-app` requires reading the `stdout` which is currently not set up to understand JSON.

Support for this should be added if we want to be able to debug a DevTools app when using `--machine` (for example, to debug a DevTools instance embedded in VS Code or intelliJ).

从 `dt serve` 入口开始，跟踪 `--machine` 如何格式化输出，以及 `--run-app` 如何读取 stdout。确认两个 flag 预期的机器可读 JSON 流程，然后验证 DevTools 应用能否在该模式下进行调试，包括所述的 VS Code 或 IntelliJ 场景。

由索引模型根据 Issue 内容生成。