fix(esbuild): mjs extensions in esm build

[Anber]

Motivation

ESM should have mjs as a file extension.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: f6d367b3cd998587805f0407d2293441f906b35f

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR