Directory as pgn source

[likeawizard]

Allow a directory to be passed as pgn source.

1. Using FileInfo identify if the resource is a directory.
2. Scan the directory for supported file formats.
3. files should be of the following format: <filename>.<pgn>[.<bz2|zst>]
   1. This means we can have a mix of files and formats.
   2. Generate a list of valid sources found in the directory
   3. Create the appropriate reader depending on file format.
   4. No recursive directory traversal. Only look at files directly in the directory and ignore child directories