feat: \nonumber/\notag support, \tag per row of {align} (#2952)

* feat: \nonumber and \notag support

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

* Remove duplicate mention of align*

* Working version of \tag within {align}

* Simpler subparse mechanism

* Fix flow errors, clarifying set-to-undefined

* Document that \tag works in rows

* Add screenshot tests

* Add Jest tests

* Add Safari screenshot

* Commit message about fixing \tag

Fixes #2379

* Apply suggestions from code review

Co-authored-by: ylemkimon <y@ylem.kim>

* Revise and move getAutoTag

* Fix handling of split

* Remove unnecessary feedTokens

Co-authored-by: ylemkimon <y@ylem.kim>

Co-authored-by: ylemkimon <y@ylem.kim>

test/katex-spec.js

@@ -3606,6 +3606,19 @@ describe("\\tag support", function() {
         expect`\tag{1}\tag{2}x+y`.not.toParse(displayMode);
     });
 
+    it("should fail with multiple tags in one row", () => {
+        expect`\begin{align}\tag{1}x+y\tag{2}\end{align}`.not.toParse(displayMode);
+    });
+
+    it("should work with one tag per row", () => {
+        expect`\begin{align}\tag{1}x\\&+y\tag{2}\end{align}`.toParse(displayMode);
+    });
+
+    it("should work with \\nonumber/\\notag", () => {
+        expect`\begin{align}\tag{1}\nonumber x\\&+y\notag\end{align}`
+        .toParseLike(r`\begin{align}\tag{1}x\\&+y\nonumber\end{align}`, displayMode);
+    });
+
     it("should build", () => {
         expect`\tag{hi}x+y`.toBuild(displayMode);
     });

test/screenshotter/ss_data.yaml

@@ -23,7 +23,13 @@ AccentsText: |
     \text{\.I İ} & \text{\H e e̋} & \text{\i ı}
     \end{array}
 Align:
-    tex: \begin{align}a &= 1 & b &= 2 \\ 3a &= 3 & 17b &= 34\end{align}
+    tex: |
+      \begin{align}
+        a &= 1 & b &= 2 \\
+        3a &= 3 & 17b &= 34 \\
+        \tag{$\ast$} a &\neq b \\
+        \nonumber a &={}? & b &={}? \\
+      \end{align}
     display: 1
 Aligned: |
     \begin{aligned}