Expose SymbolKind in hover extensions

[fwcd]

This should make it easier for clients to disambiguate identifiers. The {symbolKind} parameter uses the Show representation of this data type:

data SymbolKind = ValueFunction
                | ValueConstructor
                | Module
                | TypeData
                | TypeNew
                | TypeClass
                | TypeAlias
                | TypeVar
                | Unknown
    deriving (Show, Eq)