Enable easy passing of environment variables in browser.script.iffe.js

[ledsun]

This change is intended to meet the expectations of #645 (comment).

Summary

• Add browser.script.iife.js support for passing Ruby VM environment variables via a data-env attribute
• Add a browser Ruby::Box example that enables RUBY_BOX=1 through data-env
• Cover data-env and the Ruby::Box example with Playwright tests, and document data-env in the cheat sheet

Notes

• data-env:
  • It passes environment variables at VM startup, so supporting both WASI Preview 1 and WASI Preview 2 is appropriate.
  • There is no particular reason to exclude WASI Preview 2 from data-env support.
  • The integration test confirms that environment variables are also passed on WASI Preview 2.
• Ruby::Box example:
  • It is a useful practical data-env example because Ruby::Box requires RUBY_BOX=1 at VM startup.
  • Ruby::Box's require hook copies extension libraries to /tmp before loading them.
  • This works with the WASI Preview 1 browser shim.
  • The WASI Preview 2 browser component setup does not currently initialize writable /tmp, so the Ruby::Box example does not work there yet.
• CI behavior:
  • Ruby 3.x is skipped because Ruby::Box is not available there.
  • Ruby 4.0+ / head with WASI Preview 2 is also skipped because writable /tmp is not currently provided.
• Future work:
  • To make Ruby::Box work on WASI Preview 2, the Preview 2 browser filesystem needs a writable preopen equivalent to /tmp.
  • That is outside the scope of this data-env PR.