Merge #4113 #4136 #4141 #4142

4113: Support returning non-hierarchical symbols r=matklad a=kjeremy

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

4136: add support for cfg feature attributes on expression #4063 r=matklad a=bnjjj

close issue #4063

4141: Fix typo r=matklad a=Veetaha



4142: Remove unnecessary async from vscode language client creation r=matklad a=Veetaha



Co-authored-by: kjeremy <[email protected]>
Co-authored-by: Benjamin Coenen <[email protected]>
Co-authored-by: veetaha <[email protected]>