Handle re-selection consistently for horizontal block moves

[yukunlin]

Changes

• Removes the reliance on `[ to go the start of the shifted selection to simplify the re-selection logic.
• Preserve cursor position on MoveBlockLeft and MoveBlockRight

Info

I realized that PR #34 didn't address the root cause of the problem it's trying to fix.

The root cause is that P in virtualedit=all mode behaves inconsistently; the cursor sometimes moves one column right after pasting, other times it doesn't. This makes it difficult to rely on `[ to determine the start of the shifted selection.

PR #34 attempted to work around the inconsistencies but couldn't handle all cases:

With this patch:

[matze]

Wow, great investigation and patch!