Add a dev script to link src files into game dir

The default directory for the link script is $project_root/MonGirlDreams. A different dir can be used with -d $dir. Gitignore all MonGirlDreams* dirs to compare multiple copies of the game.

In the future, this script can be expanded with file tracking or a watch service to prevent drift.

Introduce mise to manage dev tools and tasks.