required function argument handling

I have a `helper.libsonnet` file containing this:
```jsonnet
function(a, b)
{ sum: a+b }
```
I get a "Missing argument error" which is correct but quite useless, how can I give the language server a useable default without having to resort to writing that:

```jsonnet
function(a = 0, b = 0)
{ sum: a+b }
```

it works but I don't want these arguments to be optional, is there a way to do that in a way on visible to the language server maybe ?

Reproduce the missing-argument diagnostic using the helper.libsonnet example and trace how the language server handles Jsonnet function arguments. Determine whether a separate language-server hint can be provided while arguments remain required; done means the diagnostic is more useful without changing the function's required-argument behavior.

Written by the indexing model from the issue text.