Use explicit extensions systematically

Hi,

I've noticed that `format: ["cjs", "esm", "iife"],` and `format: ["cjs", "esm"]` leads to different extensions being used. In the first scenario, `.js` is the ES Module. In the second scenario, `.js` is the CommonJS file.

This behaviour can be confusing, all the more that if you add `type: "module"` to a package.json, it changes the way `.js` files are interpreted. If you enable it, they are considered ES Modules (so a CommonJS export with extension `.js` will fail at import), if you disable it that's the contrary.

I lost a few hours trying to grasp all those concept, and would have avoided the issue altogether with more explicit extensions.

It would probably be better to be always explicit, with `.cjs` for the CommonJS export and `.mjs` export. Or if it's a breaking change, make this an option.

I don't know if this behaviour comes from Tsup or Esbuild, so I can open an issue at Esbuild if necessary.