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$ npm outdated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
```

The issue mentions vendored node_modules and an npm outdated report; start by reviewing the repository's dependency setup and the listed outdated packages. Determine the appropriate Dependabot configuration for these dependencies, with the work done when Dependabot is set up to track and propose dependency updates.

索引モデルが issue の本文から書いたものです。