Add flatten-dir plugin for importing directories

[CNSeniorious000]

Awesome Contribution Checklist:

• [x] I have read, and re-read the Contributing Guidelines
• [x] I have searched to ensure the suggested item doesn't exist on this list
• [x] This PR contains only one item

Please Provide a Link A Repository for Your Addition

https://github.com/promplate/rollup-plugin-flatten-dir

Please Describe Your Addition

This package can import a directory into module whose default export is a Record<string, string>. The keys are file pathes and values are file contents.

This format is used in StackBlitz's SDK for submitting files onto its workspace. With this rollup plugin, people can easily embed StackBlitz demos in their rollup/vite websites.

Besides, it is easy to load files into Pyodide's filesystem with this plugin, which is also good to make use of vite's HMR feature to work with Pyodide.

Here is a simple demo for its functionality: