Fix ESM output

[buzinas]

During my Vite investigation, I realized that this package's ESM output wasn't working correctly.

Although we were renaming index.js to index.mjs on the build:esm script, the imports weren't working for two reasons:

• All files should have a .mjs extension, otherwise the file is considered a CommonJS module.
• Even if I renamed all files to .mjs, it wasn't working, because the imports inside the files need to use the extension.

In order to ease the process, I just added tsup which supports outputting both formats.

I already published the package as v0.2.2 so I just need a review in order to merge this.

[treycucco]

Thanks for making these changes Vitor! I pulled in the branch locally and ran the build and everything looks as expected.