JATS writer: fix XML output for nested figures

Subfigures are now wrapped inside a `<fig-group>` element. Furthermore,
figure content that isn't allowed as children of `<fig>` elements, such
as raw text, gets wrapped in `<p>` elements to ensure schema-conform
XML production.

Fixes: #11342

test/command/11342.md

@@ -0,0 +1,33 @@
+Subfigures should be converted to *fig-group* elements.
+
+```
+% pandoc -f latex -t jats
+\begin{figure}[H]
+  \begin{subfigure}
+    \centering
+    \includegraphics[height=1.5in]{assets/A.png}
+    \caption{Graph for function *A*.}
+    \label{fig:A}
+  \end{subfigure}
+  \begin{subfigure}
+    \centering
+    \includegraphics[height=1.5in]{assets/B.png}
+    \caption{Graph for function *B*.}
+    \label{fig:B}
+  \end{subfigure}
+  \label{fig:graphs}
+  \caption{Function graphs.}
+\end{figure}
+^D
+<fig-group id="figU003Agraphs">
+  <caption><p>Function graphs.</p></caption>
+  <fig id="figU003AA">
+    <caption><p>Graph for function *A*.</p></caption>
+    <graphic mimetype="image" mime-subtype="png" xlink:href="assets/A.png" />
+  </fig>
+  <fig id="figU003AB">
+    <caption><p>Graph for function *B*.</p></caption>
+    <graphic mimetype="image" mime-subtype="png" xlink:href="assets/B.png" />
+  </fig>
+</fig-group>
+```