BUG: dist/dayz.esm.js is invalid ES module - mixes `import` and `require()`

[wmadden]

The compiled output of the package includes require() statements, which are invalid in ES modules. There's no obvious good reason for this but it's probably related to the scattering of require() statements throughout the codebase, which aren't being transpiled correctly.

See dist/days.esm.js:74:

var moment = require('moment'); // an event duration describes how an event is displayed.