Added new IGotoFile provider ExternalFile to enable opening any file in ...

In the current state this extension doesn't seem to support navigation to files in the external folder if they aren't supported language files.

I added an IGotoFile provider which allows navigation to any file within the external folders (not recursive).

On my current project we have sql migration scripts out of the solution and need to easily open them in the VS Editor for preview/updating.

Ideally this functionality would've slotted more easily into the existing resharper code but I couldn't find an existing IOccurence that was able to display the files from the external folder and also open them in the editor.

It caches the file list itself  rather than using any existing resharper cache as it seemed they were more appropriate for caching the PSI related information. There is probably a better way to do this but I couldn't easily see how to use the existing caches.