Add source maps to esm build

[Balearica]

The .min.js build includes source maps but the .esm.min.js build does not. These should be added.

[Balearica]

Unfortunately, simply adding sourcemap: true to rollup.esm.mjs did not work. Adding the rollup-plugin-sourcemaps did resolve, however this package unfortunately is not maintained so this dependency may cause issues in the future. Along with the fact that the ESM build uses rollup while the others do not, I would prefer a different solution for the ESM build if one presents itself.

Had to use this workaround due to rollup-plugin-sourcemaps specifying old versions:
maxdavidson/rollup-plugin-sourcemaps#125 (comment)

[Balearica]

Resolved by #763.