Support DocumentSymbol results from Roslyn

[davidwengier]

Our custom message target currently returns SumType<DocumentSymbol[], SymbolInformation[]>? but our endpoint blindly assumes SymbolInformation[]? is the return type. This is a bad thing because Roslyn can return either type, based on a client capability, and even the spec notes:

Servers should whenever possible return DocumentSymbol since it is the richer data structure.

This PR adds support for DocumentSymbol to our langauge server, and tests that cover both possible result types.

[ryzngard]

Our custom message target currently returns SumType<DocumentSymbol[], SymbolInformation[]>? but our endpoint blindly assumes SymbolInformation[]? is the return type. This is a bad thing because Roslyn can return either type, based on a client capability, and even the spec notes:

Servers should whenever possible return DocumentSymbol since it is the richer data structure.

This PR adds support for DocumentSymbol to our langauge server, and tests that cover both possible result types.

Ah good catch. When I wrote the original it seemed like they only returned one type