Add lang/mujs.

MuJS is a lightweight Javascript interpreter designed for embedding in other
software to extend them with scripting capabilities.

MuJS was designed with a focus on small size, correctness, and simplicity.

It is written in portable C and implements ECMAScript as specified by
ECMA-262. The interface for binding with native code is designed to be as
simple as possible to use, and is very similar to Lua.

(nia)