## Context

This PR adds an `Unbind` action, as well as syntax sugar in the keymaps
for declaring it
```
{
  "unbind": {
    "tab: "editor::AcceptEditPrediction"
  }
}
```
Is equivalent to
```
{
  "bindings": {
    "tab: ["zed::Unbind", "editor::AcceptEditPrediction"]
  }
}
```
In the keymap, unbind is always parsed first, so that you can unbind and
rebind something in the same block.

The semantics of `Unbind` differ from `NoAction` in that `NoAction` is
treated _as an action_, `Unbind` is treated as a filter. In practice
this means that when resolving bindings, we stop searching when we hit a
`NoAction` (because we found a matching binding), but we keep looking
when we hit an `Unbind` and filter out keystroke:action pairs that match
previous unbindings. In essence `Unbind` is only an action so that it
fits cleanly in the existing logic. It is really just a storage of
deleted bindings.

The plan is to rework the edit predictions key bindings on top of this,
as well as use `Unbind` rather than `NoAction` in the keymap UI. Both
will be done in follow up PRs.

Additionally, in this initial implementation unbound actions are matched
by name only. The assumption is that actions with arguments are bound to
different keys in general. However, the current syntax allows providing
arguments to the unbound actions. Both so that copy-paste works, and so
that in the future if this functionality is added, keymaps will not
break.

## How to Review

- The dispatch logic in GPUI
- The parsing logic in `keymap_file.rs`

## Self-Review Checklist

<!-- Check before requesting review: -->
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Added support for unbinding key bindings from the default keymaps. You
can now remove default bindings you don't want, without having to
re-declare default bindings that use the same keys. For instance, to
unbind `tab` from `editor::AcceptEditPrediction`, you can put the
following in your `keymap.json`
```
[
  {
    "context": "Editor && edit_prediction",
    "unbind": {
      "tab": "editor::AcceptEditPrediction"
    }
  }
]
```