* Allow paper core import in TypeScript
Typings were missing when importing paper core version with:
`import * as paper from 'paper/dist/paper-core'` syntax.
This changes the generated TypeScript definition so that it exports two
modules: `paper` and `paper/dist/paper-core`. In the same logic,
`paper-core.d.ts` file is added to make sure that the corresponding
definition is automatically loaded.
This also takes care of the fact that `PaperScript` class is not
available in paper core version, by removing it from the corresponding
TypeScript definition.
Finally, this also simplifies existing definition by directly exporting
a `PaperScope` instance as the module instead of duplicating all
`PaperScope` properties and methods on the module itself.
Closes#1713
