Add map context layers

[PaulioRandall]

What does this change?

Adds:

• BoroughsContextLayer component for boroughs and their boundaries.

Why?

Because context layers are standard across apps, especially boundary layers, so they can be optimized into concise components.

How is it tested?

Storybook & live applications.

How is it documented?

Storybook

Are light and dark themes considered?

No.

Is it complete?

• [x] Have you included changeset file?
• [x] If this adds a new component, is it exported via index.js?