Add simple elixir plugin

[clessg]

Summary

Adds a very basic elixir plugin to simplify installation to a simple devbox add elixir. Also now properly stores Mix/Hex artifacts in .devbox/virtenv.

How was it tested?

• Tested it against the existing elixir example in the repo. Seems to work fine.
• Tested on my own projects as well, separately on Mac + Ubuntu.

[Lagoja]

Hey @clessg, this PR looks really helpful!

Just wondering -- why does the plugin install CoreServices on darwin systems? Does this fix an issue you were seeing with Elixir and Devbox?

[clessg]

Hey John, thanks for the response! So happy to see Devbox is alive and well, I can't live without it now :)

Anyway, yup! I should have mentioned the Darwin part. On Mac/Darwin, the Elixir package file_system (https://hexdocs.pm/file_system/readme.html) fails to install, seemingly because it requires native libraries to compile. That package is used by Phoenix (Elixir's main web framework) and a few other frameworks for live-reloading in the browser. Here is the error message produced:

(devbox) ➜  mix phx.server

18:18:14.316 [info] Compiling file system watcher for Mac...
==> file_system
In file included from c_src/mac/cli.c:2:
In file included from c_src/mac/cli.h:4:
c_src/mac/common.h:5:10: fatal error: 'CoreServices/CoreServices.h' file not found
#include <CoreServices/CoreServices.h>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
In file included from c_src/mac/compat.c:1:
c_src/mac/compat.h:16:10: fatal error: 'CoreServices/CoreServices.h' file not found
#include <CoreServices/CoreServices.h>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
In file included from c_src/mac/main.c:1:
c_src/mac/common.h:5:10: fatal error: 'CoreServices/CoreServices.h' file not found
#include <CoreServices/CoreServices.h>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

18:18:14.697 [error] Could not compile file system watcher for Mac, try to run "clang   -framework CoreFoundation -framework CoreServices -Wno-deprecated-declarations c_src/mac/*.c -o priv/mac_listener" manually inside the dependency.

Adding CoreServices seems to resolve the issue. Maybe there's another way but I haven't found it. Considering Elixir almost always goes in tandem with Phoenix (a la Ruby and Rails) or another live-reloading framework, I figured I'd throw in that fix to minimize unexpected errors on Mac.

[Lagoja]

Thanks for the explanation! Going to merge and will test further in the next pre-release