Use dependency-group for "dev" and other non-user-facing dependencies

Nobody has claimed this yet.

`pyproject.toml` specifies two types of dependency sets: "extras" and "groups." The first is specified by `[project.optional-dependencies]` (this is what we have now), and the second is specified by `[dependency-groups]`. Extras may be installed with the `package-name[extra-name]` syntax, while groups are not supposed to be user-facing, are not listed on PyPI, and may be installed with the `--group` flag.

I propose making "dev" a dependency group in the template and changing CI/setup scripts to use it this way. This will make the extras list cleaner.

https://packaging.python.org/en/latest/specifications/pyproject-toml/#dependencies-optional-dependencies
https://packaging.python.org/en/latest/specifications/dependency-groups/

**LSDB at PyPI**

Start by inspecting the template's pyproject.toml along with the CI and setup scripts that install the dev dependencies. Update the template so dev and other non-user-facing dependencies use [dependency-groups] and ensure those scripts use the group mechanism; done means the extras list no longer contains dev dependencies and the generated project setup remains functional.

Written by the indexing model from the issue text.