feat: upgrade to a more modern build system

[lsorber]

Changes:

1. Configure a modern build system based on Hatchling v1.26, which adds support for core metadata v2.4.
2. Remove the package = true uv setting, since it's not necessary if you specify a build system:

   While uv usually uses the declaration of a build system to determine if a project should be packaged, uv also allows overriding this behavior with the tool.uv.package setting.

This is both a fix and a feature: uv's default build system is apparently still setuptools, which does not support some of the more modern core metadata features such as license-file(s). The missing feature can cause publishing to fail—see for example this failed publish workflow in Conformal Tights. It's recommended to use Hatchling while we wait for uv's own build system.