Ensure callback fires only after animation is finished.

[alexmacarthur]

Description of Proposed Changes

Due to poor Promise handling, the callback function passed to slide-element was firing without waiting for animations to fully complete.

[github-actions]

Size Change: +50 B (+3%)

Total Size: 1.9 kB

Filename	Size	Change
dist/index.es.js	1.04 kB	+32 B (+3%)
dist/index.umd.js	868 B	+18 B (+2%)

_{compressed-size-action}