Handling default parameters in functions before non-defaulted parameters

[badboy]

This is split out from #1840 and accompanies #1658

The question is how to handle this UDL:

i32 default_first(optional i32 left = 1, i32 right);

(1) not support it?

(2) Should we simply enforce argument names?

def default_first(*, left = 1, right):
    pass

(3) Only move defaulted values to the end?

def default_first(right, *, left = 1):
    pass

---

(1) is essentially what we do today. (2) is a breaking change, but probably the best option (@mhammond favored this in https://github.com/mozilla/uniffi-rs/pull/1840#discussion_r1409595660) (3) is too weird: suddenly argument order is different from what you haved defined in UDL.

Note that this is all about Python. We should clarify what happens for the other languages too.

[mhammond]

I think there's a good case to be made that this should work the same as records. cc @jeddai