New CompletionItemTag for inaccessible entities

We propose a new CompletionItemTag value to mark completion element that isn't directly visible at completion point.

For instance, in Ada "context clauses" enable visibility of entities in given compilation unit:

with Ada.Text_IO;
procedure Hello_World is
begin
   Ada.Text_IO.Put_Line ("Hello");
end;

Procedure Put_Line isn't visible without the with Ada.Text_IO; clause. But it's still desirable to have
completions for such invisible/inaccessible entities, so user is able to chose it and then add a context clause latter. IDE should highlight inaccessible completion items (for instance with gray) to let user know that this item
isn't a truly correct completion. To distinguish such items we propose a new CompletionItemTag value:

export namespace CompletionItemTag {
	/**
	 * Render a completion as obsolete, usually using a strike-out.
	 */
	export const Deprecated = 1;
	/**
	 * Render a completion as inaccessible, usually using a gray out font.
	 */
	export const Inaccessible = 2;
}

I think other languages could benefit from this too.

Start by reviewing the CompletionItemTag definition and related Language Server Protocol specification sections. Check how existing tags such as Deprecated are documented and represented, then determine the compatibility and wording requirements for an Inaccessible tag. Done means the protocol proposal is resolved and the corresponding specification changes are agreed.