feat: \nonumber/\notag support, \tag per row of {align}

[edemaine]

Support \nonumber (and equivalent \notag) using a global macro \@eqnsw to track whether one occurs in each row, similar to how amsmath uses global \@eqnswtrue/\@eqnswfalse. Fix #2950.

I also added support for \tag on each row of an {align} or {align*} or similar environment, so each row is either automatically numbered (represented by true), not automatically numbered (represented by false), or has an explicit tag (represented by an array of parse nodes). Fix #2379.

This required a more general mechanism for expanding a macro (\df@tag) that doesn't actually appear in the source code. We previously did this in parseTree.js only after parsing everything, but now we need it mid-parse too. The new Parser.subparse does the job.

I also had to add a mechanism for undefining a macro value. TeX represents this as setting a macro to \relax, but undefined was already used in some places in Namespace.js so I used that.

I added basic documentation and tests.

Example:

The tags are center-justified instead of the correct right-justified, but that was an existing bug that we can look at in another PR.

[codecov-commenter]

Codecov Report

Merging #2952 (9d1cad2) into main (40109f6) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2952      +/-   ##
==========================================
+ Coverage   93.46%   93.50%   +0.03%     
==========================================
  Files          89       89              
  Lines        6577     6616      +39     
  Branches     1524     1537      +13     
==========================================
+ Hits         6147     6186      +39     
  Misses        399      399              
  Partials       31       31              

Impacted Files	Coverage Δ
src/parseNode.js	73.33% <ø> (ø)
src/parseTree.js	100.00% <ø> (ø)
src/Namespace.js	95.23% <100.00%> (+0.36%)	⬆️
src/Parser.js	96.00% <100.00%> (+0.09%)	⬆️
src/environments/array.js	98.03% <100.00%> (+0.12%)	⬆️

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[edemaine]

Hmm, perhaps I should try to implement \tag-per-row at the same time; this should be doable with a similar (if not the same) mechanism.

[edemaine]

OK, I've added support for \tag in rows. This was harder than expected, but not too bad. See root post for details. Now ready for review.

[edemaine]

I fixed the merge conflicts here, so this is ready to review again. FWIW, we've had a recent explicit request for this feature (#3183), and #2379 has 5 thumbs-up. PR #3214 also currently depends on this one. Let me know if you'd rather I split this up into multiple PRs.

[ylemkimon]

LGTM. Sorry for the delayed review. I think subparse logic can be improved later.

[edemaine]

@ylemkimon Thanks for the nice improvements! Should be ready to look at again.

[KaTeX-bot]

🎉 This PR is included in version 0.15.1 🎉

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