Adopt new DocumentSymbol API

[jrieken]

With the 1.25 release we have finalised the API for the outline tree. A DocumentSymbolProvider should now return document symbols which can represent a hierarchy and that can differentiate between the full range of a symbol and the selection range, e.g. the name of a function.

Note that we synthesise trees using range-containment when a provider returns SymbolInformation-objects and that that isn't perfect. Adopting the new API makes the outline more robust and support features like follow-cursor and diagnostics association.

Let me know if need more information is needed. Happy Coding!