Add `.devcontainer` config for quickstart

首先生成一个 vanilla quickstart，并将其与链接的 electric-sql-quickstart 示例进行 diff，重点关注提议的 .devcontainer 配置和 /db/util.js。验证生成的应用可以在 devcontainer 中重新打开并使用 yarn start 启动；可选配置还应保留 quickstart 在容器外的正常行为。

Hey there,

I'm not sure if this is the right place to open a feature request for the quickstart template, but it would be awesome if you added an optional devcontainer configuration for the quickstart. For folks interested/willing to use the devcontainer config, getting started would be a simple as

1. npx create-electric-app@latest my-app
2. Open my-app in vscode
3. Respond to vscode prompt to "reopen in devcontainer"
4. yarn start

For my own purposes, I created a devcontainer config for the quickstart which you can find here. I also added pg-admin to the devcontainer, so someone can open their browser to http://localhost:5051 to access pgadmin and see their quickstart pg databases.

I don't expect you'll want to use the setup exactly as I have it in my example repo, as I made some small changes just to get things working that make it so that I don't think the quickstart would work outside of the devcontainer (e.g. /db/util.js -> fetchAppName() now just grabs the appName env variable, which is present in the devcontainer but might not be outside of the devcontainer). But hopefully all the heavy lifting has been done for you in case you are interested in adding a devcontainer. E.g. create a new vanilla quickstart folder and diff the output vs my example repo to see the relevant changes.

Another benefit of adding the devcontainer which I haven't tested, but theoretically Github Codespaces should "just work" if someone opens my example repo in a new Codespace--no downloading locally required.