Add paste_all_selections_before paste_all_selections_after to replicate kakoune <a-p> <a-P>.

[DerSaidin]

This is building on top of https://github.com/helix-editor/helix/pull/4694 with the following changes:

• Rebased to current head
• Choose a more meaningful primary index selection.
  • Note that this behaviour diverges from kakoune: in kakoune the resulting primary selection is always just the last selection added at the last input selection. This implementation will choose the last selection added at the current primary selection.
• Added integration test

This is an alternative to https://github.com/helix-editor/helix/pull/13600 Imo this is better because:

• it preserves separate selections
• it is a more atomic operation, not mixing in logically separate behavior (inserting newlines)
• This can be used as a building block to achieve the same outcomes as https://github.com/helix-editor/helix/pull/13600 but the reverse is not true.