Add a .gitignore in migration folder for sea-orm-cli migrate init

[sanihaq]

Please add a .gitignore with the default rust ignore pattern.
I've twice committed the target folder already

[billy1624]

Hey @sanihaq, thanks for the suggestion!! I think it would be better if we able to mimic similar behaviour of cargo init command.

If you are not in an existing git repo, cargo init does start a git repo and adds a .gitignore. If you are already in a git repo maybe change the root .gitignore to include something like */target/*

rust-lang/cargo#9633 (comment)

We can check if the migration folder resides in a Git VCS or not with this: https://docs.rs/git-discover/latest/git_discover/fn.upwards.html