Fix ESM exports in package.json. Update microbundle.

[fedemp]

Since this package is published as commonjs, es6 files are now published using the mjs extension.

Added more explicit file paths in the exports field, pointing to a unique declaration file for each bundle.

Declaration file is copied using the mts extension so it's explicitly marked as es6.

The package now passes the publint and Are the types wrong? tests.