Calling flatpickr() twice, reinitializes flatpickr

[pmishev]

https://jsfiddle.net/bwghe3v6/

Calling flatpickr(el) on an element that already has a flatpickr instance, should return that instance, not create a new one.
Relying on checking private properties such as ._flatpickr doesn't seem like a good practice, and it has other problems as well - see #2977

Your Environment

• flatpickr version used: 4.6.13
• Browser name and version: Chromium 120.0.6099.71