Add support for `InitVar`

[mthuurne]

The InitVar annotation defines a value that is passed as an argument to __init__() and then to __post_init__() but does not exist as a field on the dataclass instance. This is roughly the opposite of init=False, where the field does exist but there is no matching argument of __init__().

As there is an __init__() argument, we could actually bind values to it. But the Binder specialization needs to be extended to support it.

[mthuurne]

The template formatting would also need to include InitVar arguments in its output.