Moodle word count does not match other software

This follows on from MDL-46256.

It seems that all other systems have a consensus about how many words there are in tricky strings like

4+4=8 i.e. O(1) a,b,c,d I'm black&blue_really

All of

• Microsoft Word
• Libre office
• The OU's custom word-count atto plugin, which does the counting in JavaScript
• https://wordcounter.net/ (which I just found as the top hit on a Google search)

Consider the above string to be 6 words. Moodle does not. I think we should bring Moodle in line.

Because people who use mod_assign are the ones most likely affected by a change here, I posted in the Assign forum: https://moodle.org/mod/forum/discuss.php?d=417795