Support replacing selection with paste in Visual mode

[ulyssa]

The current InsertAction::Paste behaviour is to paste register contents before or after the cursor. However, when pasting in Vim's Visual mode, the register contents should replace the current selection. I'm going to make InsertAction::Paste take a new PasteStyle type, which will allow specifying what type of behaviour to do:

• PasteStyle::Cursor: Paste contents before the cursor, forcing the contents into TargetShape::CharWise
• PasteStyle::Side(MoveDir1D): Paste contents before or after the current selection, respecting the register's shape
• PasteStyle::Replace: Replace the current selection with the register contents, forced into TargetShape::CharWise

This will also help with representing Kakoune's p, P, and R keybindings in #32.