Offer GitHub Pages-hosted embedded store for testing

To improve onboarding and testing the store:

In case people don't have an easy way to upload HTML files for their store, we could put a template HTML page to GitHub pages. Then users would need to "whitelist" github.com with FastSpring, and that's it.

The package would need to inject the `STORE-ID` and `PRODUCT-ID` into the HTML in that case, though. Might as well inject 100% of the HTML into an empty page in that case.

首先检查 package 当前对 store HTML 的处理方式，以及托管在 GitHub Pages 上的 template 如何适配 macOS component。定义注入 STORE-ID、PRODUCT-ID 和页面内容所需的行为，然后确定 onboarding 和测试将如何验证托管流程。

由索引模型根据 Issue 内容生成。