Switch to plurimath gem

In order to reduce code duplication in projects, extract logic to another gem. It looks like the most up-to-date version is here: https://github.com/metanorma/stepmod-utils/blob/728bd50bf609afd6c7ef0a6848f45a8419a57819/lib/stepmod/utils/html_to_asciimath.rb.

- [ ] Extract HTML math conversion to a separate gem.
- [ ] Use that gem in glossarist/iev-data.
- [ ] Transfer relevant open issues from this project to plurimath/html2math.
- [ ] Consider making some performance improvements in plurimath/html2math.

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Extracted from #144:

> @skalee we have copied of the 'fake math conversion' code to here:
> https://github.com/metanorma/stepmod-utils/blob/728bd50bf609afd6c7ef0a6848f45a8419a57819/lib/stepmod/utils/html_to_asciimath.rb
>
> And this is probably time to extract out this 'fake math conversion' functionality to a separate gem under the Plurimath umbrella. Can you help with that? Thanks.

Start with the copied implementation at lib/stepmod/utils/html_to_asciimath.rb in stepmod-utils and compare it with the HTML math conversion used by glossarist/iev-data. Review the plurimath/html2math project and the related open issues before deciding the extraction boundary. Done means the separate gem is used here, relevant issues are transferred, and any justified performance work is addressed.

Written by the indexing model from the issue text.