Add syntactic support for T<string>::class

[thekid]

Example (when using https://github.com/xp-lang/xp-generics):

$name= T<string>::class; // "T\xb7\xb7\xfestring"

[thekid]

It would be great to normalize the various type members to all contain lang.ast.Type instances instead of using strings in certain situations. However, this would be a BC break and is thus out of scope for this PR.

[thekid]

Released in https://github.com/xp-framework/ast/releases/tag/v11.2.0