pagination: Update prev and next text in default props and examples

What does this MR do?

This MR follows up on https://gitlab.com/gitlab-org/design.gitlab.com/merge_requests/1439#note_183477407

This is an update to pagination's following examples:

• Basic
• Link based pagination
• Page change event

prev-text and next-text props for these examples have been added to match our design specs

Before	After

Additionally, after some discussion below, the component's default props have been updated to include the full previous/next labels to avoid confusion when using the component.

/cc @gtsiolis