Ability to inherit multiple classes

I think being able to extend multiple classes would improve usability quite a bit!

The following is currently possible:

```
class Parrot extends Pigeon {
name = "Parrot"
diet = "Berries"
extinct = false

function say() = "Pkl is great!"
}
```

It would be useful to also be able to do something like:

```
class Parrot extends Pigeon, Pet {
name = "Parrot"
diet = "Berries"
extinct = false

function say() = "Pkl is great!"
}
```

Start with the issue's single- and multiple-class inheritance examples and trace the language's inheritance implementation and related tests. Define the supported multiple-inheritance behavior and consider the work complete when the shown form is accepted with appropriate coverage.

Written by the indexing model from the issue text.