Allow analyzers to process .fsi files

[dawe]

Is your feature request related to a problem? Please describe.
Currently, .fsi files are filtered out before analyzers can process them.

Describe the solution you'd like
Remove the filter and let the analyzers decide what they want to do regarding .fsi files.

Describe alternatives you've considered
Some kind of configuration to control the filtering. But seems overkill for such a simple thing.

P.S. Of course, I don't expect other people to do this work for me. It's more a note to myself, to keep track.