feat: esm support

[tinchoz49]

Not ready to merge: working on tests

This PR add the type keyword in the package.json and package every function with the package.json generated.

Allows you to use esbuild with format: 'esm' without having to change the outputFileExtension to mjs that it doesn't work well with serverless sdk: serverless/serverless#11308

Most of the time with this change + a banner to support require it's enough to have full support for esm.

esbuild.config.cjs

...
format: 'esm',
banner: {
    js: 'import { createRequire as topLevelCreateRequire } from \'module\';\n const require = topLevelCreateRequire(import.meta.url);',
  },
...

related: #483