Ensure that the `vim::visual::Vim::visual_block_motion` method is called
when `scroll_editor` is called while in Visual Block mode. This fixes an
issue where, after getting into `Visual Block` mode, if the user used
`ctrl-d` or `ctrl-u` to scroll half of the page, the wrong selection
would be made and, if `shift-i` was used to start inserting at the start
of the line, a single cursor would be placed at the cursor position
where the selection was started, instead of one cursor per each of the
selected lines.

These changes ensure that we now match Neovim's behavior for the same
flow.

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Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>
Co-authored-by: dino <dinojoaocosta@gmail.com>