Configurable default Panel visibility

## Feature Request

### Problem

The Panel (Terminal, Output, Debug Console, Problems, etc.) does not have a setting to control its default visibility on startup.

Users can already configure the default visibility of the Secondary Side Bar via:

```json
"workbench.secondarySideBar.defaultVisibility": "visible"
```

There is currently no equivalent setting for the Panel. Users who prefer to begin with the Panel open (for example, to have the integrated Terminal immediately available) cannot configure this as part of their default workbench layout.

### Proposed Solution

Add a new setting:

```json
"workbench.panel.defaultVisibility": "visible"
```

with the same semantics as `workbench.secondarySideBar.defaultVisibility`, supporting the following values:

- `"hidden"`
- `"visible"`
- `"visibleInWorkspace"`
- `"maximized"`
- `"maximizedInWorkspace"`

This would mirror the existing `workbench.secondarySideBar.defaultVisibility` behavior and only affect the initial workspace layout.