fix: better assertions on function parameters

[sebffischer]

baby steps, but solves #95

There is still some stuff open but for that I think I need to dig a little deeper into the parser (#95)

[dgkf]

This is looking great.

There is still some stuff open but for that I think I need to dig a little deeper into the parser

Personally, I think this is probably ready to be incorporated with just the bit of polish as mentioned above, but let me know when you feel like it's complete for merging.

I think we can also address #95 here in a pretty straightforward way, but I might be overoptimistic in assuming we can just check for a missing "key".

[sebffischer]

Thanks, your suggestion worked! :) From my side it is good to be merged.

[sebffischer]

Also I appreciate your quick responses and kind feedback! This makes it fun to contribute here 🥳

[dgkf]

Just did some light touch-ups to the MR:

• Added check for when the rest-args experiment is enabled, in which case the ..args should be treated like an ellipsis parameter
• Added a few new error variants so that we have more explicit error types. Although not implemented yet, my hope is that by having more strictly typed errors they can be more easily localized eventually.
• Did a chore for myself by converting old localization names to ISO639 codes

Otherwise everything was in great shape already. Thanks for working on this!