Fix Helix `o`/`O` behavior when a selection is active.

This updates `InsertLineAbove` and `InsertLineBelow` to use the
selection bounds correctly for Helix selections, including line
selections whose end is represented at column 0 of the following line.

It also adds Helix select-mode keybindings for `o` and `O`, and adds
tests covering both line selections and selections created via `v`.

Closes #43210

Release Notes:

- helix: Fixed insert line above/below behavior when a full line is
selected

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Co-authored-by: dino <dinojoaocosta@gmail.com>