chore: Rename `extra` dependency group

[dangotbanned]

Description

This guy has been bugging me for a while:

narwhals/pyproject.toml

Lines 89 to 91 in 200baa7

	extra = [ # heavier dependencies we don't necessarily need in every testing job
	"scikit-learn",
	]

It leads to very confusing uv run commands, like this:

uv run  --extra dask ... --group extra pytest src 
#       ^^      |        ^^      |
#               |                |       
#               |        [dependency-groups]        
# [project.optional-dependencies]

Fix?

1. Use a name that doesn't match another option in uv commands
   i. or at least a dependency-related one
2. Make sure that name is self-descriptive