Include shards-core as a dependency

[Yeggstry]

Instead of this being on my notepad, this is a running list of changes that would be need to be made in order to include shards-core into this project if it is decided to do so.

• Remove src/js/shared - math and date utils are included in shards-core package.
• Update imports - math and date utils will need an import from @ingress-shards/shards-core to use the math and date functionality.
• Remove luxon as a dependency - shards-core now uses temporal-polyfill instead, and the date utils have been updated to reflected that.
• .npmrc - add ingress shards registry server (GitHub packages) and the npm auth token (GITHUB_TOKEN) to retrieve the package. See the GitHub documentation and .yarnrc.yml for more details.
• release.yml - ensure that "packages: read" is included as a permission for the GitHub Action to retrieve the dependency.