Setup Dependabot in Repo

Since there are vendored in node_modules, it might make sense. Currently there are some dependencies that are behind:
```
$ npm outdated
Package           Current  Wanted  Latest  Location
octicons            5.0.1   5.0.1   8.5.0  government.github.com
primer-core         6.2.0  6.10.9   7.0.0  government.github.com
primer-marketing    5.2.0   5.4.1   7.0.0  government.github.com
```