Fix usage for decorated class properties

[remcohaszing]

Given the following tsconfig.json:

{
  "compilerOptions": {
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true
  }
}

The following will currently causes an import error:

import { IconName } from '@fortawesome/fontawesome-common-types';

class MyClass {
  @MyDecorator
  icon: IconName;
}

This is because in some situations TypeScript doesn’t know that IconName is a
only exists as a type. It will generate an import, which can’t be resolved.

Simply adding an empty JavaScript file, will make the environment able to import
the package, even though the import value of IconName will be undefined.

I understand that:

• I'm submitting this PR for reference only. It shows an example of what I'd like to see changed but
  I understand that it will not be merged and I will not be listed as a contributor on this project.

[tagliala]

Hi!

Thanks for being part of the Font Awesome Community and thanks for this PR.

@robmadole @mlwilkerson could you please take a look?