I reported that bug with the virtualedit to the upstream Vim maintainers
and it has since been fixed in both Vim and Neovim. However, since we
will need to still support Vim <= 8.2 for a while, I also improved the
workaround that we use.

It turns out that virtualedit is only bugged in the cases where we don't
actually need to use it. Therefore, instead of fixing the cursor after
it ends up in the wrong place, we can instead disable virtualedit when
it is not needed. One advantage of doing it this way is that the `[ and
`] marks now always point to the right place after the operation.

Because of the g:loaded_move guard, we don't need to worry about
functions being defined more than once.

If we take advantage of the fact that "x" in visual mode works the same
as "d" in visual mode, we can put all the logic for moving horizontally
in a single place.

This commit removes the special case for linewise visual mode in the
ModeBlockHorizontally function. I think that it is more consistent if
all the visual modes behave the same: now it always switches to visual
mode and then tries to move the selected rectangular area.

That said, most of the time trying to move a linewise selection isn't
super useful. It only does something interesting if
move_past_end_of_line is enabled.

This makes the function more similar to MoveHorizontally.

In this commit we replace all our uses of virtcol() by col(), which
fixes a couple of bugs related to tabs.

One bug was in move_past_end_of_line. Since strwidth treats tabs as
having 1 character, if the line had tabs we would think that the end of
line appeared sooner than it should. If we tried to move something to
the right it would get "stuck" before reaching the end.

Another bug was that MoveRight needed a count of at least 8 to be able
to move over a tab. In the fixed version a count of 1 is enough, which
makes th MoveRight binding behave more similarly to "l".

Finally, there was the minor issue that "normal! 0|" is actually an
invalid command. It does the right thing because 0 moves to the first
column and then the | is ignored but relying on a command being silently
ignored is a bit fishy. Anyway, now that we don't call `|` at all this
problem is avoided entirely.

As suggested by code review, remove all blank lines after `function` and
before `endfunction`.

Following a suggestion from code review.