Consider adding the `collapsing`/`expanding` pane states when collapsing a pane using the resizer

[Zollerboy1]

Describe the feature in detail (code, mocks, or screenshots encouraged)

Hey! I'm trying to use a css transition to animate when a pane collapses/expands. Because I don't want to have the transition normally (i.e. when resizing panes without collapsing them), I only add a transition-property: flex-grow during the collapsing/expanding states:

<Pane
  collapsible={true}
  {collapsedSize}
  {minSize}
  class="transition-none data-[pane-state=collapsing]:transition-[flex-grow] data-[pane-state=expanding]:transition-[flex-grow]"
  bind:this={pane}
></Pane>

This works fine when collapsing/expanding the pane programmatically (i.e. in the example above when calling pane.collapse()). However, it does not work when dragging the pane resizer until the pane collapses, probably because this doesn't set the pane state to collapsing/expanding? It would be great if this were possible!

What type of pull request would this be?

Enhancement

Provide relevant links or additional information.

No response