Add Firefox notes for maction

[fred-wang]

Summary

Add Firefox notes for maction, taken from MDN. For that purpose, also introduces data for the selection attribute.

Test results and supporting details

• Notes taken from https://developer.mozilla.org/en-US/docs/Web/MathML/Element/maction#gecko-specific_notes
• The selection attribute is generally used together with the actiontype="toggle", so copy existing data to determine whether selection is supported.

Related issues

MDN notes are removed in https://github.com/mdn/content/pull/18594

[fred-wang]

cc @queengooborg

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.