Support element composition parsing

[nonplus]

https://github.com/google/closure-templates/blob/master/documentation/reference/element-composition.md

{template example2 kind="html<div>"}
  <{example3()}></> // Element composition call
{/template}

Right now there is a parsing error on </>: Closing tag matches nothing.

FWIW, this error might be coming from the underlying (IntelliJ) HTML template data language. No parsing error is reported if the composition tag is closed with </{}>, but that's not valid soy.