Naming convention for spin internal template parameters

Spin provides a number of special internal parameters that can be used in templates:

    async fn special_values(&self) -> HashMap<String, String> {
        let mut values = HashMap::new();

        let authors = crate::environment::get_authors().await.unwrap_or_default();
        values.insert("authors".into(), authors.author);
        values.insert("username".into(), authors.username);
        values.insert("project-name".into(), self.options.name.clone());
        values.insert(
            "output-path".into(),
            self.relative_target_dir().to_string_lossy().to_string(),
        );

        values
    }

There are two possible improvements that could be made to the parameter naming conventions:

1. A spin-* prefix could be employed to make it clear to template authors that these parameters are provided by the spin framework e.g. spin-authors.
2. project-name could be improved by being made more general e.g. template-instance-name or item-name. project-name doesn't make sense when spin add is being run b/c you're actually creating a template not an application/project.

Please discuss whether these changes would be useful/worth the effort.

Note: See #1096 for the discussion that spurred this issue.

Start with the special_values function shown in the issue and review the discussion in #1096. Determine whether a spin-* prefix and a more general replacement for project-name are warranted, then document the agreed naming convention and identify the affected template parameters.