Add settings.yml file which creates defaults for OSS repos which extend it

[mikemimik]

Adding a .github/settings.yml file to this boilerplate will allow projects to get a set of default settings. These settings will be inherited from the npm/.github repo via the _extends key in the settings.yml file.

Caveat:

Currently we are in the process of renaming the repository from github to .github in order to get some other GitHub default magic. This will change the value of _extend from "github" to ".github"

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This repository will serve as a base "default" repo for settings projects. We currently have a .github repo which is the base github org defaults. Which means any repo created under the npm org will by default use some of the functionality defined by this .github repository. (Notably: issue and pull-request templates)

By virtue of this being the "default" for open source projects, the .github/settings.yml file should be flushed out and fully formed, rather than just few items that are in there now. (Notably: branch protection, and repository defaults/settings)