Support multi-inheritance of interfaces

[blurrcat]

Typescript allows an interface to extend multiple other instances. Currently tplant doesn't support this case.

As an example, given:

interface Shape {
  color: string;
}

interface PenStroke {
  penWidth: number;
}

interface Square extends Shape, PenStroke {
  sideLength: number;
}

tplant currently generates the following which is incorrect:

@startuml
interface Shape {
    +color: string
}
interface PenStroke {
    +penWidth: number
}
interface Square extends Shape {  // should extend both Shape and PenStroke
    +sideLength: number
}
@enduml

This change produces correct PUML for such cases.

[bafolts]

@blurrcat been a while! I will update this to get it merged.

[blurrcat]

Just rebased - only conflict is the version number in package.json.