Add pageless component variation for gl-pagination

A pageless variation of the gl-pagination component is currently required for cursor pagination.

For this type of pagination, we only require two props:

page[before]
page[after]

Since results in GitLab normally come back in a block of 20, we don't require the page[size]

This component should look similar to the compact version of our pagination component.

From gitlab!21136 (comment 258386211), here's how we'd like to do the implementation:

1. Accept String types for the prev-page and next-page properties: this would let us pass cursor values directly
2. The compact mode should not require a value
3. When prev-page and next-page are specified, the corresponding buttons should simply emit their values instead of emitting an internally computed value based on the current page