Support thumbnails for packages

Description

Templates on the Typst Universe have thumbnails, which greatly help with distinguishing between them and getting a notion of what they do and how they look at a glance. I think it would be helpful for packages to have thumbnails too; currently they all have the same crate (<div role="img" class="package-icon"></div>) as their thumbnail.

Ideas for guidelines:

• Thumbnails should be clear even in small sizes.
• They should depict a logo or a representative cropped shot that demonstrates concisely what the package does, maximising the relevant information to clutter ratio.
• Care should be taken not to use shocking colours and patterns: draw attention to your package by providing good functionality and documentation, not by having a flashy thumbnail.

(Issue copied from the Forum)

The issue provides design guidelines for package thumbnails but names no implementation files, entry points, or tests. Start by locating the current package icon rendering and the existing template thumbnail support; done means packages can provide suitable thumbnails that display correctly at small sizes and are covered by appropriate tests.