Vim: enable sending multiple keystrokes from custom keybinding (#7965)

Release Notes:

- Added `workspace::SendKeystrokes` to enable mapping from one key to a
sequence of others
([#7033](https://github.com/zed-industries/zed/issues/7033)).

Improves #7033. Big thank you to @ConradIrwin who did most of the heavy
lifting on this one.

This PR allows the user to send multiple keystrokes via custom
keybinding. For example, the following keybinding would go down four
lines and then right four characters.

```json
[
  {
    "context": "Editor && VimControl && !VimWaiting && !menu",
    "bindings": {
      "g z": [
        "workspace::SendKeystrokes",
        "j j j j l l l l"
      ],
    }
  }
]
```

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/command_palette/Cargo.toml

@@ -28,6 +28,7 @@ ui.workspace = true
 util.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true
+postage.workspace = true
 
 [dev-dependencies]
 ctor.workspace = true