Feature: create partial generators

Use plop features to create incremental generators.
For example:

- A generator for adding a story to a component that already exists and has stories.
- A generator for adding an unit test to a component that already exists and has tests.
- A generator for adding `export { default } from './whatever/path/MyComponent'` to the end of a file.

The issue names no files, tests, or entry points, so first inspect the existing Plop generators and how they create React components, Storybook stories, and Jest tests. Done means partial generators can add a story, unit test, or export to an existing component-related file without recreating the whole component.

Written by the indexing model from the issue text.