Add create_new method

[cetra3]

This adds a create_new method to create a tag from scratch. It also fixes an error whereby the tag type is specified, but it still tries to read it from the path, and fails.

Resolves #35

[pinkforest]

Hey thanks - could we add a test pretty pls 🙏

Cc/ @Serial-ATA 🍏 thoughts ?

[Serial-ATA]

This would probably be better as an associated function:

impl Tag {
    pub fn empty(tag_type: TagType, config: Config) -> Box<dyn AudioTag + Send + Sync> {
        // ...
    }
}

Just to avoid having an extra error variant.