ZJIT: Add iongraph-generating Ruby script (ruby#15466)

Run like so:

    $ ../tool/zjit_iongraph.rb ../build-dev/miniruby --zjit-call-threshold=2 tmp/ghbug.rb
    false
    false
    tmp/ghbug.rb:3:in 'Object#doit': this shouldnt ever be nil (RuntimeError)
            from tmp/ghbug.rb:10:in '<main>'
    W, [2025-12-09T11:00:32.070382 #67400]  WARN -- : Command failed with exit status 1
    zjit_iongraph_67405.html
    $

Then open zjit_iongraph_67405.html with your browser.

Author: tekknolagi