Add/fix internal linting

[bmish]

Fixes a bunch of issues:

• Add lint script to run internal linting
• Add linting config with eslint / eslint-plugin-eslint-plugin / eslint-plugin-node / prettier
  • These are the plugins recommended by: https://eslint.org/docs/developer-guide/working-with-plugins#linting
  • Uses existing prettier configuration in .prettierrc
• Autofixed lint violations
• Fixed some inconsistencies with TypeScript usage
• Add npm keywords to package.json recommended by: https://eslint.org/docs/developer-guide/working-with-plugins#share-plugins

It looks like test/linting aren't running on CI, so separately GitHub Actions should be added to run those.

[willhoney7]

LGTM! Thank you!