[API Proposal]: GlobDirectories, Glob and exclude patterns

### Background and motivation

`Pathy.Globbing` only exposes `GlobFiles`. Two capabilities that `Microsoft.Extensions.FileSystemGlobbing` already provides are not reachable: matching directories, and excluding patterns. Exclusions in particular are close to essential in real projects, because almost every glob over a source tree needs to skip `bin`, `obj`, `node_modules` or `.git`. Today callers have to post-filter the results, which is both slower and more verbose than letting the matcher do it.

### API Proposal

```C#
namespace Pathy
{
public static class ChainablePathGlobbingExtensions
{
public static ChainablePath[] GlobDirectories(this ChainablePath path, params string[] globPatterns);
public static ChainablePath[] Glob(this ChainablePath path, params string[] globPatterns);

public static ChainablePath[] GlobFiles(this ChainablePath path, string[] include, string[] exclude);
}
}
```

`Glob` matches both files and directories. The `include`/`exclude` overload maps directly onto the matcher's `AddInclude` and `AddExclude`.

### API Usage

```C#
var sourceFiles = ChainablePath.Current.GlobFiles(
include: new[] { "**/*.cs" },
exclude: new[] { "**/bin/**", "**/obj/**" });

var testProjects = ChainablePath.Current.GlobDirectories("**/*.Specs");

var everything = (ChainablePath.Current / "artifacts").Glob("**/*");
```

### Alternative Designs

* A small options object or builder instead of an `include`/`exclude` pair. More extensible, but heavier for the common case.
* Leave exclusion to LINQ after the fact. That is what people do today, and it means walking directories that are then thrown away.

### Risks

Two positional string array parameters are easy to transpose, so named arguments should be encouraged in the documentation, or the shape reconsidered. Adding an overload next to the existing `params string[]` version may create ambiguity at the call site and needs checking.

Locate the existing ChainablePathGlobbingExtensions.GlobFiles implementation and its use of Microsoft.Extensions.FileSystemGlobbing. Compare the proposed GlobDirectories, Glob, and include/exclude overload against the current API; done means the requested APIs match the examples, directory and exclusion matching works, and documentation covers named arguments.

Written by the indexing model from the issue text.