Support returning non-hierarchical symbols

If `hierarchicalDocumentSymbolSupport` is not true in the client capabilites
then it does not support the `DocumentSymbol[]` return type from the
`textDocument/documentSymbol` request and we must fall back to `SymbolInformation[]`.

This is one of the few requests that use the client capabilities to
differentiate between return types and could cause problems for clients.

See https://github.com/microsoft/language-server-protocol/pull/538#issuecomment-442510767 for more context.

Found while looking at #144

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