Support directory references or glob patterns for `<Folder>` in `.slnx`

## Problem Statement
In `.slnx` format, Solution Folders (``) require every non-project file to be explicitly enumerated using individual `` elements:

```xml

```

When referencing external folders, documentation trees, test suites, or companion codebases (e.g., C/C++ trees or scripts alongside .NET projects), maintaining static file lists creates substantial maintenance overhead whenever files are added, moved, or deleted.

## Proposed Solution
Enhance the `.slnx` schema and parser to support directory-level inclusion or globbing patterns for solution items.

### Option A: Directory reference with recursive inclusion
```xml

```

### Option B: Globbing / wildcard syntax (consistent with MSBuild project items)
```xml

```

## Benefits
1. **Zero-Maintenance Directory Inclusion**: External companion directories, shared assets, documentation trees, and C++ codebases stay automatically synchronized in the Solution Explorer.
2. **Consistency with SDK Projects**: MSBuild SDK-style projects (`.csproj`, `.fsproj`) already default to recursive wildcard includes (`**/*`); bringing similar capability to `.slnx` makes solution item management much cleaner and more intuitive.
3. **Cleaner Solution Files**: Dramatically shrinks `.slnx` file size and avoids merge conflicts on large teams.

Start by reviewing the existing .slnx schema and parser handling for Folder and File elements. Compare the proposed directory-reference and glob-pattern options, then define their inclusion and path semantics before implementing one. Done means the chosen syntax is supported for solution items and its behavior is covered by tests.

Written by the indexing model from the issue text.