Use last token in type name for #nullable context

[cston]

Use the last token in the type name for the #nullable context. For instance, use the ] for arrays and > for constructed types.

using System.Collections.Generic;

class Program
{
#nullable disable
    string
#nullable enable
        [] names; // string~[]!

    Dictionary<string,
#nullable disable
        string>
#nullable enable
        map;      // Dictionary<string!, string~>~
}

[jcouv]

@cston @AlekseyTs I think that was already implemented by Aleksey in #31607
If there is a remaining issue, could you clarify it?
Otherwise, please close

[jcouv]

@cston Do we need to keep this issue open?