Update GetDeclarationListSymbols to expose FSharpSymbolUse

This allows the display context to be used during formatting so the current open statements are taken into consideration.

**Note:** I had to move `FSharpSymbolUse` further up `service.fs` so that `FSharpSymbolUse` could be referenced by `x.GetDeclarationListSymbols`
Change _denv to denv now it's actually being used :)