Add .gitignore from .hgignore and also ignore unity generated files

[Schroedingers-Cat]

The repo is missing a .gitignore causing a lot of files to be shown as added that shouldn't be added to git. This PR adds a .gitignore from the content of the .hgignore already present as well as some additional ignores for files that'll be generated when opening the project with the Unity editor.

[janmunity]

Thanks!