Migrate block cursor to Layer and RectangleMarker API

[masad-frost]

https://github.com/replit/codemirror-vim/blob/master/src/block-cursor.ts copied the implementation of codemirror's selection, somewhere around this version https://github.com/codemirror/view/blob/0.19.27/src/draw-selection.ts. Most of the magic has since been moved to a public API https://github.com/codemirror/view/blob/6.16.0/src/draw-selection.ts#L4

I think using those primitives will simplify the implementation and make it more maintainable.