Add a VisualBlock mode

Instead of trying to extend the Mode::Visual special case, just split
out into three different modes.

crates/vim/test_data/test_visual_delete.json

@@ -1,7 +1,7 @@
 {"Put":{"state":"The quick ˇbrown"}}
 {"Key":"v"}
 {"Key":"w"}
-{"Get":{"state":"The quick «brownˇ»","mode":{"Visual":{"line":false}}}}
+{"Get":{"state":"The quick «brownˇ»","mode":"Visual"}}
 {"Put":{"state":"The quick ˇbrown"}}
 {"Key":"v"}
 {"Key":"w"}