Properly support tagging

fitch.sty is marked as incompatible with tagging (https://github.com/latex3/tagging-project/issues/1485). The main issue was that fitchproof used a list environment to control the alignment; and that's been changed to flushleft (commit a90efaa20186480fd3b1e7c7b79627216cc34daf). So now a fitchproof doesn't create a wrong tagging structure (a table inside a list with one item) but something better.

That doesn't mean that fitchproofs now are properly tagged when tagging is on. The scope lines are coded as \rules and produce unknown Unicode characters in the tagging structure. More has to be done to do this properly, but it has to wait until the tagging framework is more stable (and documented).

To produce an actually accessible document (i.e., something a low vision reader can actually use with a screen reader) the best option is (and I suspect will remain) conversion to HTML, as done, e.g., for https://github.com/rzach/forallx-yyc. See https://richardzach.org/2023/07/converting-latex-to-html-technical-notes/ for what's required/been done for forallx. (I used LaTeXML to produce HTML for Fitch proofs with added structural descriptions as described in https://forallx.openlogicproject.org/html/A4.html#S0.SS0.SSS0.Px3)

Start with fitch.sty and the tagging-project issue 1485, then review commit a90efaa20186480fd3b1e7c7b79627216cc34daf to understand the existing alignment change. Wait for the tagging framework to stabilize and become documented before defining the implementation. Done should mean fitchproof produces a usable tagging structure without unknown Unicode characters from its scope lines.