Add in_preview boolean context for keybindings

[krawitzzZ]

Body

What are you proposing?

Add an in_preview boolean context to Zed's key binding system that indicates when the current editor is in preview mode. This would allow users to create context-specific keybindings that only apply to preview tabs.

Why does this matter?

Currently, Zed has a preview tab system where files opened from the project panel or via "go to definition" automatically open in preview mode (displayed with italic tab names). However, there is no way to create keybindings that are specific to when an editor is in preview mode.

This feature would enable users to:

• Bind keys to actions only when in preview mode (e.g., q to close the tab)
• Create different keybindings for preview vs. regular tabs
• Customize their workflow for quick navigation and cleanup of preview tabs

Without this, users cannot create context-specific bindings like Editor && in_preview, which limits customization options for a commonly used feature.

Are there any examples or context?

VS Code Comparison:
VS Code has a similar feature with workbench.editor.enablePreview and exposes an isPreview context for keybindings. Users can bind keys like:

{
  "key": "q",
  "command": "workbench.action.closeActiveEditor",
  "when": "editorTextFocus && isPreview"
}

Zed's Current State:

• Preview tabs already exist and work well
• The Pane struct tracks preview state via preview_item_id
• The dev:OpenKeyContextView command shows active contexts, so users could verify the new context
• Other boolean contexts exist: menu, renaming, in_snippet, showing_completions, etc.

Example Use Case:
With this feature, users could add to their settings.json:

{
  "keybindings": [{
    "context": "Editor && in_preview",
    "bindings": {
      "q": "pane::CloseActiveItem"
    }
  }]
}

This would allow pressing q to close preview tabs (like clicking a file from the project panel or following "go to definition"), while regular tabs would remain unaffected.

Possible approach

The implementation would be a small addition to the key_context_internal() method in src/editor.rs. The editor would check if it is the preview item in its containing pane and add the in_preview context to the key context.

The logic:

1. Get the editor's entity ID from window.current_view()
2. Get the workspace via self.workspace()
3. Find the containing pane via workspace.pane_for_item_id() (public method)
4. Check if this editor is the preview item using pane.is_active_preview_item()
5. If yes, add "in_preview" to the key context

This is a minimal change (~9 lines of code) that follows existing patterns in Zed's codebase for boolean contexts.