flatpickr doesn't provide an easy possibility to set the today's date.
To solve this issue, the plugin `shortcut-buttons-flatpickr` is added
with a configuration for the today's date.

Resolves: #100635
Releases: main, 12.4
Change-Id: I882f05ab4486d1b3073008cadb1d0275dd77c8ce
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79867


Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>