Code for constraint accordion was copy-pasted before previous
improvement. Old version is still in use for Segments. When we get to
improving constraint editing we should rebuild segments editing, without
use of this code.
This PR creates/steals the logic and basic components that we need for
the new constraint editing design and shows it instead of the old one if
the flag is on.
The interface needs a lot of work, but this essentially wires everything
up so that it works with the API on direct editing:
<img width="781" alt="image"
src="https://github.com/user-attachments/assets/97489a08-5f12-47ee-98b3-aefc0b840a2b"
/>
Additionally the code here will need a lot of refactoring. This is a
first draft where I've yanked all the constraint editing logic out of a
nested hierarchy of components that handle validation and lots more. I
expect to clean this up significantly before finishing it up, so please
excuse the mess it's currently in. It turns out to have been lots and
lots more logic than I had anticipated.
This is just a PR to get started, so that the next one will be easier to
work on.
Implements the new design for playground constraints. They're not in use
in segments yet, and strategy parameters have not been touched. This PR
establishes a pattern that we can follow for strategies and parameters
later.

The PR also includes a change in how the constraint item organizes its
children: it now takes care adding padding and spacing itself, instead
of the children doing that. It looks right most places, but segments
aren't quite right anymore. However, as this is behind a flag, I'd
rather fix that in a separate PR.
---------
Co-authored-by: Tymoteusz Czech <2625371+Tymek@users.noreply.github.com>
- new way of showing strategy variants
- fixed wrapping issue in strategy editing, for a lot of variants
defined (`SplitPreviewSlider.tsx` change)
- aligned difference between API and manually added types