Support creation of non-terminals with type aliases

[shadaj]

This is helpful for things like identifiers, where we only have a single field in the struct but want to share a common leaf definition. Ideally we could support:

#[rust_sitter::leaf(...)]
type Ident = String;

so that users can interact directly with the string. This is a bit tricky though, since we can't add an impl for the type alias.