Deals with https://github.com/zed-industries/community/issues/772
Closes
https://linear.app/zed-industries/issue/Z-1518/allow-keybindings-to-be-removed

Now, configuration like 
```json5
[
    {
        "context": "Editor",
        "bindings": {
            "alt-v": null,
        }
    }
]
```

will make `alt+v` to print `√` instead of moving the caret one page up.

Release Notes:

- Added a way to disable keybindings with `null` value