ESM modules aren't registered in package.json

**Describe the bug**
Even though this package comes with an ESM version, it's not clear to me how build tools are supposed to pick up on it, when the ESM modules aren't specified anywhere in the package.json. Out of 'main', 'module' and 'exports' only the 'main' field is filled out and it links to day.min.js, which is not ESM.

**Expected behavior**
Package.json should be updated to point to the modules inside `./esm/` through the `module` and `exports` fields

**Information**
irrelevant

Start by inspecting package.json and the ESM modules under ./esm/. Check how the existing main field points to day.min.js, then verify that the module and exports fields identify the ESM entry points; the work is done when package metadata clearly exposes those modules to build tools.

Written by the indexing model from the issue text.