Visual selection utility fixes

[mehalter]

While playing around with the visual selection functionality I found a couple bugs.

1. When using visual line mode the functionality was just broken. Mainly because when visual line mode is used, the end_col and start_col will be out of bounds of the line. This makes sure to only trim the lines if the start_col and end_col are in the bounds of the lines
2. With the new ability to get the visual mode to pass into update_prefills I noticed that the current method for "leave visual mode" actually puts you into selection mode in the already open instance of grug-far which is very disorienting. This changes the method of leaving visual mode before getting the selection so that when updating the prefills you end up in the mode you expect in the grug-far window. While fixing this, I also normalized the use of this functionality to a utility function so there is a single place to change how to "leave visual mode"
3. Adds a second function get_current_visual_selection_split. This is useful internally and could be useful for the end user.
4. Refactors the code so that all locations where the visual selection is retrieved it is gotten through get_current_visual_selection_split so the logic is centralized and easier to maintain in the future and to populate bug fixes across all locations in the code.

Let me know what you think!

[MagicDuck]

thank you so much for the fixes! Requested changes are mostly renames 😄

[mehalter]

Thanks so much for the review! I applied the requested changes 😄

[MagicDuck]

Only one tiny nitpick about mentioning strict param in help doc. Otherwise all good!

[mehalter]

Ah, nice catch. Just added the parameter to the docs

[MagicDuck]

great!