asdf plugin for just

For my development and testing workflow I use the asdf version manager. It lets you install multiple versions of the same tool at once and change the used version on the fly.

I also needed the tool just, which is like make, but more for automation and less for compiling.

But as the current plugin for just was not working, i wrote my own one and created a pull-request in the official plugin repository. repository of asdf.

The PR was quickly merged, and now it is an official asdf plugin.