Read additional files from QGIS project file

[fgravin]

This is a proposal to slightly improve the developer experience while using the qgis-js API.

At the moment, we have to list all the files that are used within the QGIS project, beside the QGIS project file itself. eg.

const source =
  "/Glacier_3857";
const files = ["glaciers.qgs","glaciers.gpkg", "glaciers_attachments.zip"];

It would be great if the additional files could be guessed by the API by reading the QGIS project file.