feat(intro_page): allow skipButton and backButton to be used at the same time

[AtjonTV]

While using your package we had the requirement to show the skip button on the first screen, while showing the back button on the others.

It only shows the skip button on the first page when skip and back are enabled at the same time.

Internally we have been using this patch since October 2023 without issues. I am currently trying to bring our custom patches into upstream projects, so that is why this PR is opened now.