dmn-js: Viewers index.js: Use Viewer named exports to improve

[BrianJVarley]

Proposed Changes

Related to {I945}

• This changes includes the Viewer, NavigatedViewer & Modeler as named exports from the dmn-js library.
  Exporting these classes instead of just default has some benefits for consumers. We don't need to use the long import path import Viewer from 'dmn-js/lib/NavigatedViewer' instead import { NavigatedViewer } from 'dmn-js';. It also means you don't need to re-alias the Viewer when importing.

For example with named exports:

export { default } from './Viewer';
export { Viewer } from './Viewer';
export { NavigatedViewer } from './NavigatedViewer';
export { Modeler } from './Modeler';

Steps to reproduce the behavior:

Import NavigatedViewer using named import - import { NavigatedViewer } from 'dmn-js';
Import is undefined, only default import exists with internal path - import Viewer from 'dmn-js/lib/NavigatedViewer

[barmac]

Cf. #945 (comment)

[BrianJVarley]

Feel free to close this, as you mentioned it needs a discussion.