Provide option to set z-index

[phil-warner]

I'm using the datepicker in the context of an SPA that has multiple slide-outs with a variety of z-indexes. The datepicker defaults to a tailwind class of z-50, which is sometimes too low for what I need.

[samal-rasmussen]

Upstream vanillajs-datepicker v. 1.2.0 has a different fix that should make it unnecessary to set z-index in most cases: https://github.com/mymth/vanillajs-datepicker/blob/master/CHANGELOG.md#potentially-breaking-changes

But this fork is way behind and is missing several fixes like this.