Using gems with Node WASI

Hey, since I needed it myself, and based on https://gist.github.com/kateinoigakukun/5caf3b83b2732b1653e91b0e75ce3390, I wrote JS code that allows you to use gems with Node WASI.

Unfortunately, as you need preopens for that (to map the gems into the WASI vfs), it's currently not possible to use DefaultRubyVM for that, as its options are limited to env.

I wrote my own small wrapper to allow me access to the full breadth of WASI options: https://github.com/primate-run/primate/blob/master/packages/ruby/src/private/wasi.ts, but it took me a bit to understand how to get the RubyVM from the package to mimic the behaviour of DefaultRubyVM. The examples in the cheatsheet are all using DefaultRubyVM.

It is possible to perhaps make DefaultRubyVM a bit more flexible, so that it's possible to use preopens with it?

Alternatively, if you want to keep the defaults simple, perhaps add this example to the readme or cheatsheet? I don't mind sending a PR.

まずcheatsheetとREADMEにあるDefaultRubyVMの使用方法を読み、次にリンク先のwasi.tsラッパーとそのpreopensの扱い方と比較します。意図された結果が、より柔軟なDefaultRubyVM APIなのか、それとも文書化されたNode WASIの例なのかを確認し、gemsをWASI仮想ファイルシステムにマッピングできることを検証します。