v4.0.0

The project structure and code base underwent significant changes, resulting in the release of version 4.0.0 of the package.

Change overview:

• moved all source files to src folder
• moves dist.js file into dist folder
• refactores dateInput.js (old input.js) class into utility functions (inputUtilities.js)
• updates dev dependencies
• adds force date picker polyfill feature
• updates README.md if new paths and feature