Support complex anchors in yaml

[ChaserVasya]

_templateEnum: &templateEnum
        enumVal1: ''
        enumVal2: ''
        
....
templateUsage(context=...,param=...):
    <<: *templateEnum
    enumVal2: 'some text'

error:
type 'ObjectNode' is not a subtype of type 'StringTextNode' in type cast

[Tienisto]

The merge operator is currently not supported by the yaml: (see: dart-lang/yaml#121).

[Tienisto]

I tried out different anchors and they seem to work out of the box. I guess if the Dart team add support for the merge operator, it should also work when using slang. There is nothing we can do now (except implementing a custom yaml parser which exceeds the scope of this package).