vim visual block (#2849)

Release notes:

- vim: add Visual Block mode
([#984](https://github.com/zed-industries/community/issues/984)),
([#1415](https://github.com/zed-industries/community/issues/1415)).
- vim: add support for `a<object>` and `i<object>` in visual modes
- vim: fix scroll shortcuts (`ctrl-{f,b,d,u,e,y}`) in visual modes
- allow `shift-enter` to type a newline.

assets/keymaps/default.json

@@ -173,6 +173,7 @@
     "context": "Editor && mode == full",
     "bindings": {
       "enter": "editor::Newline",
+      "shift-enter": "editor::Newline",
       "cmd-shift-enter": "editor::NewlineAbove",
       "cmd-enter": "editor::NewlineBelow",
       "alt-z": "editor::ToggleSoftWrap",