docs: Add JS code snippets for forms (#54577)
https://nextjs.org/docs/app/building-your-application/data-fetching/forms-and-mutations
Some of the code blocks where I only specified TS code snippets aren't
working with JS as well (when there are no types). I think I might have
incorrectly assumed you could leave those out. So added the JS snippets
in here (same code, just different file names).
