Use .mjs extensions for ESM

[rtsao]

Node interprets packages as CommonJS unless package "type": "module" is declared or .mjs extension is used.

This PR resolves this problem by using .mjs extensions for the ESM bundle.

• This yields SyntaxError: Cannot use import statement outside a module when importing this package
• https://publint.dev/react-idle-timer@5.7.2

[vercel]

Someone is attempting to deploy a commit to a Personal Account owned by @SupremeTechnopriest on Vercel.

@SupremeTechnopriest first needs to authorize it.

[alexfreska]

Hey @SupremeTechnopriest thank you for the great package.

I am also hitting this issue, this package cannot be used in a package that gets imported into nextjs, here is some more context: vercel/next.js#39375 (comment) I tried manually changing to .mjs like this PR does, and it works great.