[needs-packaging] mozjs38, Mozilla's SpiderMonkey JavaScript engine

GNOME Shell and several GNOME apps are written in JavaScript and use Mozilla's SpiderMonkey JavaScript engine via gjs.

GNOME 3.22 uses mozjs24. It looks like GNOME 3.24 will use mozjs38 (skipping mozjs31). Without this update, we can't update gnome-shell to 3.24.

This switch is important because the SpiderMonkey releases correspond with Firefox releases which include security updates. The current mozjs24 corresponds with 24.2 released December 2013. The current Firefox ESR is 45.7 but the latest Spidermonkey release is 45.0.2 from April 2016: https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Releases/45

This will add a 3rd version of mozjs to Ubuntu 17.04 but the goal is to drop mozjs24 in 17.10. The ancient original mozjs package in particular is used by couchdb which has found it difficult to use more recent mozjs versions.

$ reverse-depends src:mozjs
* couchdb-bin (for libmozjs185-1.0)
* gxine (for libmozjs185-1.0)
* libproxy1-plugin-mozjs (for libmozjs185-1.0)
* oolite (for libmozjs185-1.0)

$ reverse-depends src:mozjs24
Reverse-Depends
===============
* cinnamon (for libmozjs-24-0v5)
* edbrowse (for libmozjs-24-0v5)
* gnome-shell (for libmozjs-24-0v5)
* libcjs-dev (for libmozjs-24-dev)
* libcjs0 (for libmozjs-24-0v5)
* libgjs-dev (for libmozjs-24-dev)
* libgjs0e (for libmozjs-24-0v5)

$ reverse-depends src:gjs
Reverse-Recommends
==================
* python-pocket-lint (for gjs)

Reverse-Depends
===============
* gnome-characters (for gjs)
* gnome-documents (for gjs)
* gnome-maps (for gjs)
* gnome-shell (for libgjs0e)
* gnome-shell (for gjs)
* gnome-sound-recorder (for gjs)
* gnome-sushi (for libgjs0e)
* gnome-weather (for gjs)
* ostree-tests (for gjs)
* polari (for gjs)
* ubuntu-budgie-desktop (for gjs)
* ubuntu-gnome-desktop (for gjs)
* unity-webapps-dev (for gjs)

mozjs38 will be maintained in Debian possibly by the mozjs24 maintainer once GNOME 3.24 is packaged.