Calling methods on derived types

[breml]

This issue is split from #744.

expr is not able to handle types derived from base types, which provide methods, see https://github.com/expr-lang/expr/issues/744#issuecomment-2574798824 for more details.

Example:

type say string

[antonmedv]

This is more of a feature request, than a bug) So I'm going to label it as a feature.