Chore: Replace tslint with eslint and improve typing of app

[KevLehman]

• Replace TSLint (since it's deprecated)
• Added ESLint support for app
• Added Prettier support for app
• Fixed ESLint warnings due to use of and added new type declarations

[murtaza98]

Hi, @KevLehman There are a couple of conflicts introduced here due to recent merges...Could you please take a look at them?

[KevLehman]

Updated. Please check that my resolutions are the right ones 👀

[murtaza98]

Hey, @KevLehman I've included my suggestions within this PR - https://github.com/RocketChat/Apps.Dialogflow/pull/73. Please review and merge them here when you get a chance :smile:

Thanks for this initiative :blue_heart: