[folding] Distinctly highlight corresponding `#endregion` for `#region X` comments

That would apply to all supported languages. But I considered it for Python only, and so may be missing something. In python, standing on a `#region` marker highlights all `#region` tokens in the file, as is normal for other tokens in plain source and in comments. **Request**: Apply a different highlighting rule (perhaps "line boxes" as is applied to matching brackets/parentheses/braces) to the `#endregion` corresponding to the `#region` under the cursor, to make clear the region's boundary.