Have provider invoke fn support not being given a function name

[jeffcharles]

Description of the change

Adds support to QuickJS provider's exported invoke function to handle not being given a function name. This shouldn't change any behaviour.

Why am I making this change?

As part of #768, we'll want to remove the eval_bytecode exported function (to make it easier for plugin developers) and just have invoke so invoke needs to also be able to handle eval_bytecode's use case where there is not an exported function to run.

I haven't deleted eval_bytecode in this PR because we still emit function invocations for it. I also haven't updated the dynamically linked codegen code to call invoke instead of eval_bytecode because all runtime environments using the provider would need to be updated with a provider version that includes this code change, otherwise new modules generated to use invoke with 0 arguments for function name pointer and length would fail.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-core do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.

[saulecabrera]

Left one minor comment -- not blocking, else LGTM.

[saulecabrera]

Just a minor comment here - could this be consolidated into:

pub enum UseExportedFn {
  Invoke(Option<&'static str>)
  ...
}

[jeffcharles]

Yes, I can make that change.