Spacer Block: Update UI to support editing complex height & width

[stacimc]

What problem does this address?

#36186 adds custom units to the Spacer block. Because the height and width attributes are now strings, it is also possible to set complex values for these attributes, for example:

<!-- wp:spacer {"height":"max(1.25rem, 5vw)"} -->
<div style="height:max(1.25rem, 5vw)" aria-hidden="true" class="wp-block-spacer"></div>
<!-- /wp:spacer -->

This is only possible in the block editor by editing the values in the Code Editor, as the UI is a simple UnitControl that doesn't support complex values. If a block has a complex val for height for example, the input appears empty.

It would be useful to have a way to see the value and edit it through the UI.

What is your proposed solution?

I'd love to get some design feedback on this. The simplest example would be to update the UnitControl with an edit toggle that can open a text field for entering custom values.

[stacimc]

Linking #35558 which discusses the design for supporting custom values in form fields. I'll leave this open to track the issue for the Spacer, since its height/width fields are implemented ad-hoc and not coming from a shared block support.