Add typescript definitions?

[Potherca]

I noticed the package doesn't have type declarations either, so it's possible that it's not configured properly to work with Typescript / Vite.

^{Source: #1 (comment)}

It might be worth adding Typescript definitions. The question, if so, is whether to add them here or if they should be a separate NPM package explicitly for dev requirements.

Explicitly as one of the stated goals is to keep the MetroJS package(s) themselves as light-weight as possible.