The `footnotes` extension does not work when the `superscript` extension is enabled

[DannyBen]

Description

Enabling both the footnotes and the superscript extensions causes the footnotes extension to stop working.

Environment

• Commonmarker version 1.0.0.pre6
• Ruby 3.1.3
• Ubuntu 20.4

Reproduction code

require 'bundler/inline'
gemfile { gem 'commonmarker', '1.0.0.pre6' }

markdown = <<~MARKDOWN
  Gimme footnote[^1]
  This is ^superscript^

  [^1]: Here is the footnote
MARKDOWN

extensions = { footnotes: true, superscript: true }
options = { extension: extensions, render: { hardbreaks: false } }
output = Commonmarker.to_html markdown, options: options
puts output

Actual output

<p>
  Gimme footnote[^1]
  This is <sup>superscript</sup>
</p>

Expected output

(which can be observed by disabling the superscript extension in the code above)

<p>
  Gimme footnote<sup class="footnote-ref"><a href="#fn1" id="fnref1">1</a></sup>
  This is <sup>superscript</sup>
</p>
<section class="footnotes">
  <ol>
    <li id="fn1">
      <p>Here is the footnote <a href="#fnref1" class="footnote-backref">↩</a></p>
    </li>
  </ol>
</section>

code was prettified manually, for clarity

[gjtorikian]

Super good find, thank you. This is an upstream issue: kivikakk/comrak#272