fixes: #169 Add missing d.ts file in ecs-morgan-format

[imsuvesh]

fixes #169 Added missing index.d.ts file in package.json files property so it will be added in next release.

[cla-checker-service[bot]]

💚 CLA has been signed

[david-luna]

Thanks @imsuvesh for your contribution :)

[trentm]

@elasticmachine, run elasticsearch-ci/docs

[trentm]

I'll get a release out with this today (https://github.com/elastic/ecs-logging-nodejs/pull/171).