math extension: don't fail when display math contains...

...embedded inline math.  See jgm/pandoc#7942.

Note, however, that there's a larger issue this doesn't
solve.  For in principle you could have embedded math
in inline math, e.g.

```
$\text{hi $x$ there}$
```

Pandoc's markdown parser gets this right, but that's
because it uses a LaTeX tokenizer.

commonmark-extensions/src/Commonmark/Extensions/Math.hs

@@ -55,6 +55,7 @@ pDisplayMath = try $ do
   (_, toks) <- withRaw $ many1 $
                   choice [ () <$ symbol '\\' >> anyTok
                          , noneOfToks [Symbol '$']
+                         , try (symbol '$' <* notFollowedBy (symbol '$'))
                          ]
   count 2 $ symbol '$'
   return $! displayMath (untokenize toks)

commonmark-extensions/test/math.md

@@ -37,6 +37,22 @@ e=mc^2
 \]</span></p>
 ````````````````````````````````
 
+Note that display math can contain embedded inline math:
+
+```````````````````````````````` example
+This is display math:
+$$
+\text{Hello $x^2$}
+$$
+.
+<p>This is display math:
+<span class="math display">\[
+\text{Hello $x^2$}
+\]</span></p>
+````````````````````````````````
+
+
+
 To avoid treating currency signs as math delimiters,
 one may occasionally have to backslash-escape them: