Path Contexts

We'll need a file system API for virtual paths. Local path (#1) should implement the same API, so it should be possible to abstract over both kinds of paths.

The API should allow to:
- manipulate foreign paths, e.g. Windows paths on a Linux computer, and vice versa;
- introduce fully virtual file systems.

Currently, I am thinking of adding a new field of type `IFileSystem` or `IPathContext` to each of our path types: hopefully, this won't create too much overhead for their storage.

Default constructors would rely on a singleton instance for the current system, something like `new LocalPath("foo") => new LocalPath("foo", LocalFileSystem.Instance)`.

There should be a way for the user to introduce custom path validation in constructor. For example, if they want to somehow avoid Windows paths on Linux and want to throw on `new LocalPath(CustomContext, @"C:\Windows")`, even though this path is valid.

## TODO
- [ ] Introduce a field into all `*Path` types.
- [ ] Derive the path manipulation operations to the path context:
- [ ] path separators,
- [ ] behavior aspects such as Windows drives,
- [ ] path validation in constructors (even for `LocalPath` that doesn't do any validation otherwise).
- [ ] Allow passing the context to path constructors.
- [ ] The constructors should delegate to some `LocalPathContext.Instance` by default (if nothing else is passed).
- [ ] `LocalPathContext.Instance` should be a writable field (for global custom override).
- [ ] Future enhancement request: allow `ThreadLocal` and/or `AsyncLocal` override for `LocalPathContext`, to avoid reliance on the global state.

Aucun fichier ni test n’est nommé. Commencez par localiser les *Path types et LocalPath existants, puis suivez leurs constructeurs et leurs opérations de manipulation des chemins. Le travail est terminé lorsque ces types partagent une API de contexte de chemin prenant en charge les séparateurs, le comportement propre à la plateforme, la validation, les contextes de constructeur personnalisés et le comportement par défaut de LocalPathContext.Instance décrit dans le TODO.

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