Return anyhow error instead of IO error

[jeffcharles]

Description of the change

Have plugin::new_from_path return an anyhow::Error instead of an io::Error.

Why am I making this change?

Mostly consistency with the rest of the API.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-plugin 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.