Add support for latex blocks / environments

current support for latex fragments was inline only, i.e. lines containing block
elements (e.g. a line starting with `* `, i.e. a headline) will not be parsed
as part of the latex fragment but the respective block element. Parsing latex
blocks at the block level should fix that. Note that in any case we don't do
any processing and just emit the raw latex (leaving the rendering to e.g. js).

org/testdata/latex.html

@@ -6,8 +6,18 @@ we support <code class="verbatim">\(...\)</code>, <code class="verbatim">\[...\]
 <li>\[\sum_{i=1}^n a_n\]</li>
 <li>$$\sum_{i=1}^n a_n$$</li>
 <li>\begin{xyz}\sum_{i=1}^n a_n\end{xyz}</li>
-<li>\begin{xyz}
+<li>
+\begin{xyz}
 \sum_{i=1}^n a_n
-\end{xyz}</li>
+\end{xyz}
+</li>
 <li>$2 + 2$, $3 - 3$</li>
+<li>
+\begin{xyz}
+latex block ignores block lvl elements (e.g. list - line starting with -)
+a = b
+- c
+- d
+\end{xyz}
+</li>
 </ul>

org/testdata/latex.org

@@ -10,3 +10,9 @@ we support =\(...\)=, =\[...\]=, =$$...$$= and =\begin{$env}...\end{$env}= as la
   \sum_{i=1}^n a_n
   \end{xyz}
 - $2 + 2$, $3 - 3$
+- \begin{xyz}
+  latex block ignores block lvl elements (e.g. list - line starting with -)
+  a = b
+  - c
+  - d
+  \end{xyz}

org/testdata/latex.pretty_org

@@ -10,3 +10,9 @@ we support =\(...\)=, =\[...\]=, =$$...$$= and =\begin{$env}...\end{$env}= as la
   \sum_{i=1}^n a_n
   \end{xyz}
 - $2 + 2$, $3 - 3$
+- \begin{xyz}
+  latex block ignores block lvl elements (e.g. list - line starting with -)
+  a = b
+  - c
+  - d
+  \end{xyz}