* sets up linting config and runs `yarn lint --fix` once, so that all changes will show up correctly in future PRs.
* Note that there are still a lot of linter errors.
- Defines a TypeScript type for PaymentTransaction
- Provides an optional function to users for verifying a PaymentTransaction instance at runtime: verifyPaymentTransaction()
- Adds tests for verifyPaymentTransaction()
- Adds isFlagEnabled() util to be used for models