Add a way to explicitly request the registering a (non-typedef) type alias without requesting the dictionary.

[pcanal]

As seen at cms-sw/cmssw#51761

We can have the situation where we have, the TClass::GetClass for the various spelling (myclass, Wrapper<myclass>, ShortCut) work without AutoParsing but the spelling Wrapper<OtherName> does not.

// Header in Library 1
#pragma once

struct myclass {};

template <typename T>
struct Wrapper {
   T value;
};

// Library 1
<class name="myclass" />
<class name="Wrapper<myclass>" />

// Header in Library2
typedef OtherName myclass;
using ShortCut = Wrapper<myclass>;

// Library 2
<typedef name="OtherName" />
<typedef name="ShortCut" />

What does not work is:

<typedef name="Wrapper<OtherName>" />

which makes sense since there is no such 'typedef' but there isn't yet a syntax to express it, eg:

<alias name="Wrapper<OtherName>" />

This issue would be solved by introducing the tag alias in the selection xml grammar and #pragma link C++ alias in the linkdef grammar that would register the name (indirectly) via the ::ROOT::AddClassAlternate interface and add the proper documentation and test for this new tag.

[pcanal]

This issue relates but is different to the issue #19705