Skatepark: Add "Next Post" and "Previous Post" to post navigation

[beafialho]

Currently the post navigation is looking like this:

It should look as shown in the Figma comp. Post navigation terms should be H6 and post titles should be size Extra Small 14px in Medium weight.

cc: @kjellr

[MaggieCabrera]

We didn't do this originally because the block didn't allow it, I think that has changed so we can revisit this

[pbking]

While we can add labels via block attribute since block-templates aren't yet localizable the content "PREVIOUS POST" and "NEXT POST" would instead need to leverage something like the pattern block.