atomkraft init should generate a .gitignore file

[rnbguy]

Since an atomkraft project generates many artifacts, the generated project should contain an appropriate .gitignore.

We don't need GitPython dependency. We can write the file at the project root.

Note that atomkraft still doesn't initialize the project as a git repo. The current workflow of creating a git atomkraft project repo is,

atomkraft init my_project
cd my_project
git init .