Make notebook cell output item limit (500) configurable via setting

## Problem

When a notebook cell produces many output items (e.g., repeated `print()` calls, logging, tqdm progress bars, or training loops), VS Code hard-caps the rendered output at **500 items** per cell. Once the limit is reached, a message appears:

> "There are more than 500 outputs, show more (open the raw output data in a text editor)"

This limit is **hard-coded** in the source code (`var y0e=500`) with no user-facing setting to adjust it.

### Impact

This affects many real-world use cases:
- **Machine learning training** — long-running training loops that print epoch/loss metrics each iteration
- **Data processing pipelines** — scripts that log progress for thousands of items
- **Debugging with logging** — cells mixing `print()` and `logging` that produce many output items
- **Progress bars (tqdm)** — each update can create a new output item

### Additional Issues

1. The **"show more" link** that is supposed to open the raw output often **crashes** with:
> "Unable to resolve text model content for resource vscode-notebook-cell-output"
(See #226261)

2. The `notebook.output.textLineLimit` setting only controls **text lines per output item**, not the **number of output items** — users naturally expect it to solve this problem but it doesn't.

3. Related issues: #228857, #226261, #110581

## Proposed Solution

Add a new configurable setting:

```
"notebook.output.maxItems": 500
```

- **Default**: `500` (backward compatible, preserves current behavior)
- **Range**: `1` — `10000` (or similar reasonable bounds)
- **Description**: "Maximum number of output items to render per notebook cell. Increase this value if cell outputs are being truncated."

### Implementation Details

The hard-coded limit is located in:
```
src/vs/workbench/contrib/notebook/browser/notebookEditorWidget.ts
```

The variable `y0e=500` (minified) maps to the output limit constant used in:
- `NotebookCellOutputContainer` (output renderer)
- `NotebookCodeCellWarmup` (warmup logic)
- `NotebookCodeCellViewModel` (view model slicing)

The fix would involve:
1. Registering the new setting in `notebook.contribution.ts`
2. Reading the setting value instead of using the hard-coded constant
3. Passing it through to the output renderer and related components

## Alternatives Considered

- **Just increase the limit** (e.g., to 5000): Doesn't solve the problem for power users; too high for others
- **Dynamic limit based on output size**: More complex, harder to predict behavior
- **Configurable setting**: Best balance of flexibility and simplicity ✅

## Environment

- VS Code Version: 1.100.0+
- OS: Windows 10/11
- Reproducible: Yes, any notebook cell that produces >500 output items

## Reproduction

```python
# Any simple loop that produces many outputs
for i in range(1000):
print(f"Iteration {i}")
```

This will show "There are more than 500 outputs" with no way to increase the limit via settings.