feat(scandir): Consider global gitignore when forming ignore patterns

[levouh]

Still need to do some more testing, but the idea here is to support global ignore configuration on top of whatever is found within the directories of the given project.

Definitely some added overhead here as we now have to check not only local .gitignore files, but when we find nothing there we then have to check against every pattern in the global ignore as well. In my rather naive testing, the difference is not noticeable.

Some notes:

1. Is it alright to use plenary.job here to check the existence of this file?
2. Is there a preferred way to "run a job and get the output synchronously" beyond what I've done here?
3. Is storing the global ignore under the key "" going to conflict with anyting?