Fixes InsertLineAbove (Shift + O) auto-indent handling by deciding
whether to trim the first character or the last character based on
direction. Previously, inserting a line above would auto format the line
the cursor was originally on and not the new line (instead of the
correct behavior which is to do the opposite).

No tests because I couldn't find any Vim-specific auto-indent tests and
this only affects Vim mode. If I'm missing something there LMK and I'll
take a closer look.

Closes #52588.

Release Notes:

- Vim: Fixed auto-indentation for insert above action.

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