`EnumString` should derive `From<&str>` instead of `TryFrom<&str>` when `#[strum(default)]` is present

If you derive `EnumString` on a enum with a `#[strum(default)]` it generates a `TryFrom<&str>` implementation which can never return an error. It would make sense to generate `From<&str>` instead

This would be a breaking change