Shapefile generation script should work for multiple polygons

# ✨ Feature Request

At present, the shapefile generation application - lib/ants/cli/ancil_create_shapefile.py - will only work with a single shapefile. We already know of cases where we would need to generate and store multiple shapefiles in one go - see the `cci_glacers/runme.py` script in the data repo.

## Requirements

Be able to specify multiple polygons to be created in a shapefile and save it.

## Context

Shapefile creation for cci glaciers.

Start with lib/ants/cli/ancil_create_shapefile.py to understand how the current single-shapefile flow receives polygon input and saves its output. Compare its expected usage with cci_glacers/runme.py in the data repo. Done means multiple polygons can be specified and saved in one shapefile.

Written by the indexing model from the issue text.