Adding BaseUri::unixPath and BaseUri::windowsPath

[nyamsprod]

This PR resolve #124 by adding two new methods to BaseUri:

• BaseUri::windowsPath
• BaseUri::unixPath

The conversion can only be made if there is no scheme OR if the scheme is file. For any other scheme the methods will return null.

[stof]

As my use case will involve dealing with an argument being League\Uri\Contracts\UriInterface, am I right when thinking that the usage of those new methods would look like that ?

return BaseUri::from($uri)->unixPath();

[nyamsprod]

@stof you are correct on the usage. This way any kind of URI can be used even string URI.