Add aria attributes and adjust back-ref convention

[nonoesp]

Hi, @ambroisemaupate!

This pull request adds aria accessibility role attribute to the footnotes div and footnote back-refs and changes the naming convention of refs and ids from using dashes to colons. A final step to comply with michelf/php-markdown's conventions.

• Changed footnote ref prefix from #fn- to #fn:
• Change footnote ref sup prefix from fn-ref- to fnref:
• Changed footnote id prefix from fn- to fn:
• Added footnote backref role="doc-backlink"
• Added footnote container class="footnotes" role="doc-endnotes"
• Updated tests

[ambroisemaupate]

Hello @nonoesp

Thank you so much for your pull request. Hope you’re safe. I’ll try to find a moment to review it but that seems already good.

[ambroisemaupate]

Thanks @nonoesp!

[nonoesp]

Hope you're safe as well. Thanks so much for the review and woot! for releasing 1.0.0. 🥳