Add vendor directory to AppStore build/release #65

Several users that install the Solid-Nextcloud app throuch the NC App Store run into issues caused by the vendor directory not being present. As users should not have to run composer themselves, the vendor directory needs to be part of the packaged release.

Looking at how other packages do this (for instance user_oidc and mail) the most straightforward way of doing this seems to be a combination of using GitHub Actions and krankerl.

In order to have this setup correctly, the package release needs to be automated (see #64), so this can be added as part of the automation.