[Pagination] Add label prop

[Tetsuro]

Feature request summary

Add a label prop to the Pagination component.

Rationale

Allows for a label to provide more information regarding pagination. For example, we can use it to show X of X pages.

Implementation details

• prop name: label?
• prop type: string

The aria-live attribute can be used to read out the label to screen reader users.

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[beefchimi]

For reference, this is implemented in online-store-ui:
https://github.com/Shopify/online-store-ui/pull/230

The PR should be relatively easy to follow, as Tetsuro cloned the component in the first commit, then made adjustments in all subsequent commits.