Add Ruby::Box example

[ledsun]

Summary

• add a Node.js example showing how to enable Ruby::Box with RUBY_BOX=1 when creating DefaultRubyVM
• document the new example command in the ruby-wasm-wasi example README

How It Works

Run the following commands in the packages/npm-packages/ruby-wasm-wasi/example directory.

npm install
node ruby-box.node.js

Output:

(node:7650) ExperimentalWarning: WASI is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
ruby.wasm: warning: Ruby::Box is experimental, and the behavior may change in the future!
See https://docs.ruby-lang.org/en/master/Ruby/Box.html for known issues, etc.
Ruby::Box.enabled?: true
box::X: 123