org-element: Change to export-snippet syntax
* contrib/lisp/org-element.el (org-element-export-snippet-parser):
Change syntax from @back-end{contents} to <back-end@contents>.
(org-element-export-snippet-successor): Use new syntax
* testing/lisp/test-org-element.el: Add test.
This change is required as curly braces conflict with LaTeX syntax
when trying to break a command in two parts. On the other hand, HTML
tags can be broken easily in two. More explicitely, both the
following constructs are possible now:
<latex@\textsc{>Name<latex@}> and <html@<b>>