Fix off by one error in visual mode

[Chillee]

Fixes #1755, fixes #1719, #1590, #1455, #1160

List of fundamental breaking changes:

1. Decoration instead of cursor.
2. A block cursor doesn't show when we do visual mode on an empty line.
3. When you do <d-d> and you don't have editor.selectionHighlight set to false, the carets don't appear.

It's done!

Also note that in anticipation of possible future delegation to Neovim, I have decided not to take this opportunitity to also refactor the yank/put logic.