Unhide carets in add selection for occurrence and fix error

[kitbdev]

• fixes https://github.com/godotengine/godot/issues/87310
• fixes https://github.com/godotengine/godot/issues/71217

Applies to both add_selection_for_next_occurrence (ctrl+d) and skip_selection_for_next_occurrence (ctrl+alt+d). Uses the selection from line in the search, this way if multiple lines are selected nothing happens instead of printing an error. Uses _unhide_carets(); from #86978 to make sure the carets are always visible.