Throw exception if filename contains slash

[bizurkur]

• Throw exception if vfsStreamFile contains a slash in the name.
  • This resolves vfsStream::newFile allows slashes in filenames #142.
  • This is the same behavior that vfsStreamDirectory has.

[allejo]

While we're on the topic of blacklisted characters in filenames... Should there be an array of blacklisted characters instead? I know : is a forbidden character at least on macOS and I think on Windows too.

[bizurkur]

As for other blacklisted characters, that sounds like something that should be done as a separate PR to me but possibly a good idea. I think the point of this issue was that allowing / gave the interpretation that it would automatically create a directory containing a file and that's not what happens.