Display 'default_step' property of @CommerceCheckoutPane plugins in administration

Before we implement this, what do the maintainers say?

@jacksicksack: Further thoguhts or details where and how this should be placed?

I ran into this in each commerce project where we needed to place the checkout panes in different regions.

Just had a quick chat on slack with @jsacksick if we should implement this feature. His answer:

Why not [...]

He also said:

But I'd consider this low prio TBH

I think this is particularly important for payment panes

But not sure how you can clarify this from the UI

I'd suggest we create a first MR and wait for feedback 👍

Now, the same PHPUnit error as in ✨ Allow admin_description for @CommerceCheckoutPane plugins RTBC occurs, which again seems to be unrelated to the changed made in the MR.

@lrwebks this is simply the wrong approach taken. Please use the same approach for output like you already did well in 🐛 Mapped media fields are overridden with metadata on translation save Needs review !
So I'm a bit disappointed why you hacked it this way here now...

Having a dedicated class then like there, (e.g. pane-default-step) we can then discuss how to style it less prominent. For the first step please make the font size small and the text in italics. Label and value should have dedicated classes like
<div class="pane-default-step"><span class="pane-default-step__label">Default region:</span> <span class="pane-default-step__value">main</span></div>

After wards please remove the Draft state from the MR.

I have now implemented the system in the same way as ✨ Allow admin_description for @CommerceCheckoutPane plugins RTBC as well as added CSS and a proper test. I agree, that my previous approach was definitely flawed, yet I am still convinced that the use of the t() function is necessary here, as we need a way to translate the "Default Step:" label, and concatenating other strings with a t() function that only wraps around the label violates Drupal standards and undermines the purpose of t(), see t() Concatenation Dos and Don'ts