add TypeScript declaration file

[rwalle61]

For #43

To test this I did npm pack to check that the index.d.ts is included, and used the module locally and checked that ts-jest is happy with it and VSCode reads it correctly.

Let me know if any changes are needed 🙂

[joerg-walter-de]

  type Validator = {
    validate: (
      key: string,
      payload: unknown,
      channel: string,
      operation: 'publish' | 'subscribe',
    ) => void;
  };

Return type seems to be boolean

https://github.com/WaleedAshraf/asyncapi-validator/blob/master/src/MessageValidator.js#L38

[WaleedAshraf]

@joerg-walter-de Yes. It'll return true or throw an error. Good catch. Can you open a PR to fix this?