refactor: npm discount codes

[spencern]

Resolves #6123
Impact: minor
Type: feature|refactor

Issue

Move Discount Codes plugin to npm.

Solution

Created npm package for discount code plugin
https://www.npmjs.com/package/@reactioncommerce/plugin-discount-codes

Testing

1. Run reaction in dev mode with this branch
2. Create at least one discount code via Reaction Admin
3. Using GraphQL playground and a valid authorization code with the admin role run a discount codes query such as this:

{
  discountCodes(shopId: "Your Shop Id") {
    nodes {
      _id
      discount
      transactions {
        cartId
      }
      discountMethod
      description
    }
  }
}

4. Ensure you see discount code data returned

[spencern]

Snyk license checks will fail because this PR introduces a new GPLv3 dependency. This is expected and we can merge anyway

[kieckhafer]

Works as expected 👍