Improve tech normalization of plural words

[jackfirth]

This pull request adjusts the behavior of @tech{} to better handle normalizing plural words to their singular forms. Specifically, it adds a rule to strip a trailing "es" in some circumstances and to not strip a trailing "s" if the last two letters of the term are "ss". I wanted this because @tech{syntax class} and @tech{syntax classes} currently normalize to syntax clas and syntax classe respectively.

[mflatt]

My only hesitation here is that this change can break some cross-document references. For example, someone might have rendered a paper that points to documentation at https://docs.racket-lang.org/ on the theory that tag links are stable. I don't know how big of a problem that might be, though. Any thoughts?

I've sometimes had to write things like @tech{syntax class}es to get links to work, so I can certainly appreciate the improvement.

[jackfirth]

It's possible. I don't know of a good fix for that problem. But I don't think "we can never change anything about how tag links are created" is an acceptable stalemate. Is there a way we can check that this at least doesn't break any documentation links in existing packages?

[samth]

We could just generate both tags in the HTML.