Implements LSP documentSymbol request to provide a list of all symbols
defined in the current document. Clients use it to show an outline and
allow to jump to the symbol's definition.
This implements the simple form of the signatureHelp feature, which only
displays the signature of the function being typed, without indication
of the active argument.
Active argument detection, while accounting for destructuring to support
each and for special forms turned out to be more involved than expected
and is left for a follow up patch.
