Add support for checking unsaved files, same as in XS/VS

[rojepp]

IFileSystem should be implemented, as was done in XS here: fsprojects/zarchive-fsharpbinding#602

FSAC would then keep a list of modified file contents, and present those as fact to FCS via IFileSystem.
Editors are already sending modified file contents, so implementing this would improve error checking without editors needing to change anything.

[rojepp]

I'd be happy to take a stab at this.

[rneatherway]

@rojepp This would be very welcome, thanks! I will test and review any PR you send. It's been on my list for ages, but no idea when I would get to it myself...