Add `.gitignore`

[hartikainen]

I noticed you guys don't have a .gitignore yet so I added one here. This one combines the following github-provided .gitignore files:

• https://github.com/github/gitignore/blob/main/Python.gitignore
• https://github.com/github/gitignore/blob/main/community/Bazel.gitignore
• https://github.com/github/gitignore/blob/main/Global/Patch.gitignore
• https://github.com/github/gitignore/blob/main/Global/macOS.gitignore