Missing "documentHighlightProvider": True in server_capabilities

[rcoacci]

In #43 was added documentHighlightProvider to the implementation of fortls but the server doesn't advertise this to the client, so the client doesn't send the request and we can't get symbol highlights.

Looking at the code it seems to be missing an entry in the server_capabilities dict that is sent to the client advertising the documentHighlightProvider capability. As a test, I added "documentHighlightProvider": True to that dict in my local installation and the highlight functionality started working correctly