Get information on times of use of "open"s on hover

**Is your feature request related to a problem? Please describe.**

https://github.com/fsharp/fslang-suggestions/issues/1029#issuecomment-858771098
> I often cut down considerably on the number of opens by e.g. only doing an `open System`, and then `IO.Directory.CreateDirectory` and `IO.File.ReadLines` and so on. Or it could be a single `open Microsoft.Extensions` for everything below that. It's not always practical I admit, but I don't like having opens for just one or a very few cases.

**Describe the solution you'd like**

https://github.com/fsharp/fslang-suggestions/issues/1029#issuecomment-858772809
> Would be nice if on mouse over we could see how many uses each `open` had.

**Describe alternatives you've considered**

Not doing anything.

**Additional context**

N/A