Vim visual block mode

This isn't quite an exact emulation, as instead of using one selection
that is magically in "column mode", we emulate it with a bunch of zed
multi-selections (one per line).

I think this is better, as it requires fewer changes to the codebase,
and lets you see the impact of any changes immediately on all lines.

Fixes: zed-industries/community#984

crates/vim/src/state.rs

@@ -44,6 +44,7 @@ pub enum Operator {
 #[derive(Default)]
 pub struct VimState {
     pub mode: Mode,
+    pub last_mode: Mode,
     pub operator_stack: Vec<Operator>,
     pub search: SearchState,