adding vega_datasets to list of dependencies

vega_datasets contains example datasets which are used
in the Vega-Altair gallery of examples

having this dependency would make this exercise
easier: https://aaltoscicomp.github.io/python-for-scicomp/plotting-vega-altair/#exercise-adapting-a-gallery-example

software/environment.yml

@@ -21,4 +21,5 @@ dependencies:
   - scalene
   - ruff
   - altair-all
+  - vega_datasets
   - xarray