support \llangle, \rrangle

[nschloe]

The MnSymbol package supports double angled brackets \llangle, \rrangle. Would be great to see this is in KaTeX. Could perhaps be emulated by \langle\!\langle.

MWE:

\documentclass{article}
\usepackage{MnSymbol}

\begin{document}

$\llangle\rrangle$

$\langle\langle\rangle\rangle$

$\langle\!\langle\rangle\!\rangle$

\end{document}

LaTeX output: