Update package to es6 module syntax

[jbloomstrom-wac]

Modernize Flickity Codebase to ES6 Modules

Overview:
This PR modernizes the Flickity codebase by converting it from CommonJS to ES6 module syntax. The following changes have been made to improve the maintainability and future-proofing of the codebase:

Changes:

1. Convert to ES6 Modules:

   • Replaced all require statements with import statements.
   • Used export default for module exports.

2. Updated Individual Module Files:

   • core.js: Updated to use ES6 syntax and exports.
   • drag.js: Converted the module to ES6 syntax, ensuring all functionality remains intact.
   • prev-next-button.js: Refactored using ES6 classes and modules.
   • page-dots.js: Refactored using ES6 classes and modules.
   • player.js: Refactored using ES6 classes and modules.
   • add-remove-cell.js: Updated to ES6 syntax.
   • lazyload.js: Updated to ES6 syntax.
   • imagesloaded.js: Updated to ES6 syntax.

3. Main Entry File:

   • Converted the main entry point to use ES6 imports and export the Flickity module.

[Jones-S]

I assume this was not merged. But I desperately need this 😅