chore: switch esbuild output format from iife to esm

[wojtekmaj]

What's the problem this PR addresses?

This setting currently blocks Berry from using ESM modules that use top-level await (e.g. ink).

How did you fix it?

Switching output format to esm seems to be fixing it, while the executable still works.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[arcanis]

Various things (of the top of my mind, yarn set version, whose generated file is yarn-x.y.z.cjs) expect the Yarn build to be a CJS file; we can't migrate to ESM without a major release.

[wojtekmaj]

Oh darn! I have so many improvements in my mind that would require ESM builds. Well, please consider switching to ESM in the next major then!