[Lens] Allow creating layers while dragging a field

[myasonik]

For now, this would only work from the Available Fields list but if #90441 was completed, this could apply for between layers as well.

Summary

I'd love to be able to create a new layer while dragging.

For mouse users, this could work by hovering a field over the "Add a layer" button for ~2 seconds which would create the layer allowing you to place the field you're holding onto one of the new drop targets.

For keyboard users, this could work by making the "Add a layer" button a drop target that, when activated, created a layer but didn't drop the field you're holding. Then placing you on a drop target within the new layer.

Why?

This would save a couple of clicks for mouse users (and going back across the screen) and could potentially save a lot of back and forth for keyboard users (Lens can really rack up the tab stops).

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[markov00]

In order to provide better transparency of priorities, issues that will not be prioritized within the next 24 months are being closed.

Tracking request in Lens general improvements ice box #184459