Support for typstPackages from upstream nixpkgs

[cherrypiejam]

Upstream nixpkgs currently maintains all Typst packages from Typst Universe registry, including their hashes and dependencies. This PR adds support for reusing existing Typst packages from nixpkgs. For these packages, users are no longer required to specify their hash and dependencies.

An example is also provided with this PR. Feel free to discard it or merge it into an existing example.