JavaScript support

The main goal of TypeGraphQL is to get rid of SDL schema and TS interfaces duplication in favor of one single source of truth (classes with decorators). However, after adding new features like dependency injection, validation, authorization and subscriptions, there are some benefits of using this library instead of pure `graphql-js` or `graph-tools`.

Technically, it would be possible to use TypeGraphQL with babel's `transform-class-properties` and `transform-decorators-legacy` plugins and some modification in decorators signature and logic.

It also would be nice to support TypeScript compilation with Babel 7 using `@babel/preset-typescript` and a bunch of decorators/metadata plugins.

However I'm not sure if there's a demand for this feature.
So **if you are interested**, please 👍 so I will know that I should implement it 😉

Start by reviewing the existing decorator signatures and logic, then compare the requirements of Babel's transform-class-properties, transform-decorators-legacy, and @babel/preset-typescript plugins. Done would mean documented, working JavaScript support and TypeScript compilation through Babel 7, with the required metadata and decorator behavior defined.

Written by the indexing model from the issue text.