Class Diagram: syntax for inner classes

[hattesen]

It would be really useful, if the mermaid syntax for classDiagram would allow diagramming inner classes (or enumerations).

Currently inner classes (and enumerations) have to be modeled separately, e.g.

classDiagram
      class Animal {
        -int age
        -Gender gender
        +mate()
        +age() int
      }
      Animal *-- Gender
      class Gender {
        <<enumeration>>
        MALE
        FEMALE
        OTHER
      }

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Nesting class/enumeration definitions could be achieved using a syntax like...

mermaid
   classDiagram
      class Animal {
         class Gender {
            <<enumeration>>
            MALE
            FEMALE
            OTHER
        }
        -int age
        -Gender gender
        +isMammal()
        +age() int
      }
      Animal *-- Animal::Gender