[API Proposal]: Children, Files and Directories enumeration without the globbing package

### Background and motivation

Listing the contents of a directory currently requires either the separate `Pathy.Globbing` package (and therefore a dependency on `Microsoft.Extensions.FileSystemGlobbing`) or a drop down to `ToDirectoryInfo()`. For the simple, extremely common case of "give me the entries in this directory", neither is appropriate. The core package can offer this with no new dependencies, since it is a thin wrapper over `Directory.Enumerate*`.

### API Proposal

```C#
namespace Pathy
{
public static class ChainablePathExtensions
{
public static IEnumerable Children(this ChainablePath path, bool recursive = false);
public static IEnumerable Files(this ChainablePath path, bool recursive = false);
public static IEnumerable Directories(this ChainablePath path, bool recursive = false);
}
}
```

### API Usage

```C#
foreach (var project in (ChainablePath.Current / "src").Directories())
{
Console.WriteLine(project.Name);
}

var configFiles = (ChainablePath.Current / "config").Files()
.Where(x => x.HasExtension(".json"));

var everything = ChainablePath.Current.Children(recursive: true);
```

### Alternative Designs

* Point people at `Pathy.Globbing` and `GlobFiles("*")`. That forces a package and a transitive dependency on consumers who only wanted to list a folder.
* Expose these as properties. Enumeration hits the file system and can be expensive, so methods communicate the cost better.

### Risks

Behaviour on a path that does not exist, or that points at a file rather than a directory, has to be defined: returning an empty sequence is friendlier than throwing but can hide mistakes. Recursive enumeration also has to state what happens when a subdirectory cannot be accessed.

Start by reviewing the existing ChainablePathExtensions API and the Pathy.Globbing alternative, then compare the proposal with the underlying Directory.Enumerate* behavior. The issue is not ready to implement until behavior for missing paths, file paths, and inaccessible recursive entries is decided; done would include the three methods and settled semantics.

Written by the indexing model from the issue text.