Add visibility to SymbolInformation

I propose an addition to SymbolInformation that allows an LS to report the visibility (private, protected, public) to the client:

interface SymbolInformation {
  visibility?: Visibility;
}
enum Visibility {
  Private = 1,
  Protected = 2,
  Public = 3
}

A client can choose how he uses that information:

• it could display an icon that indicates a symbol is private
• after getting a list of symbols, it could allow the user to hide/show only public symbols

The same property could get added to CompletionItem.

I think private, protected and public should cover all languages, or are there more modifiers in some languages?

The enum should be structured to allow something like

if (symbol.visibility >= Visibility.Protected) {
  // symbol is at least protected or public
}

Start with the SymbolInformation proposal in issue #98 and compare it with the suggested CompletionItem extension. Resolve whether the visibility enum covers the languages represented by the protocol and define the compatibility and ordering expectations before identifying the relevant protocol definitions and tests. Done means the design is agreed and both affected types are specified consistently.