Support defining aliases for generic definition types.

[Dogwei]

I've heard that C#12 will improve type aliases, so here's my opinion:
I hope to support defining aliases for generic definition types, example:

 using Map<TKey, TValue> = System.Collections.Generic.Dictionary<TKey, TValue>;
 using IntegerMap<TValue> = System.Collections.Generic.Dictionary<int, TValue>;

If this feature is available, it will be more convenient to rewrite code from other languages into C#.
And it can also solve the problem of generic definition type name conflicts.

[ufcpp]

see dotnet/csharplang#4452

[jcouv]

Closing in favor of dotnet/csharplang#4452 which already tracks this feature request.