feat: export `elements!` macro

[oskardotglobal]

Hello, thank you for this wonderful project.

I was looking into the project's source code when I found the elements! macro and it surprised me that it isn't public as it is quite useful when working with web components. I've added appropriate documentation and replaced the imports with imports available from outside the crate so this can be used from other crates.

Moving this from #25 because I accidentally created the PR on main.