Platform-specific methods (quoting etc.) for local paths

First of all, there are some specific APIs for paths (say, `GetCompressedFileSize` on Windows, also Unix rights and Windows ACLs) which we may seek to implement.

Second: there are several absolutely different ways to convert paths to strings. For certain Windows APIs, it's important to handle these properly (i.e. quote paths, or append a UNC prefix where appropriate, see [Vacuum's `AbsolutePath`](https://github.com/ForNeVeR/Vacuum/blob/c16aab2b8b9e164d4889a17494160dbe315df2f0/Vacuum/AbsolutePath.fs#L49-L57) as inspiration).

Obviously, not all users want the platform-specific parts of the file path API, so we may include some separate platform-specific packages for all of this.

I imagine our users calling something like
```csharp
LocalPath fsp = …;
MyNativeFuncs.DoWinApiOperation(fsp.QuoteWinApi()); // QuoteWinApi is an extension
```

Or, say
```csharp
LocalPath fsp = …;
WindowsACL acl = fsp.GetAcl(); // an extension from Win package
UnixRights rights = fsp.GetUnixPermissions(); // an extension from Unix package
```

Commencez par examiner l’abstraction LocalPath existante et l’implémentation Vacuum AbsolutePath liée, pour le quoting des chemins et la gestion de UNC. L’issue ne nomme aucun fichier ni test du dépôt ; sa finalisation nécessiterait de définir précisément le périmètre des API et des packages de chemins spécifiques aux plateformes, notamment le quoting Windows, les ACLs et les permissions Unix.

Rédigé par le modèle d'indexation à partir du texte de l'issue.