Improve Assets handler: Performance and ensuring that late registration doesnt stop enqueueing

[bordoni]

This will need to be ported to https://github.com/stellarwp/assets/ later.

Changes here are:

1. enforcing a better performance by avoiding logic for already enqueued asset.
2. Ensure that late registration doesn't prevent enqueuing/registering with WP.

Change number 2 is vital for ensuring that Assets that are registered JIT still get enqueued, example Elementor Widgets.