[Flock][Feature] - Flex() make it easier to reverse child order

[rensPols]

Flutter Issue Ticket Link:
#54821

Why didn't Flutter implement this?
No time I guess.

Why does this belong in Flock?
This is made clear in the existing issue

[matthew-carroll]

Please explain who needs this and why it's a valuable change, so that we can determine if it's worth maintenance within Flock.

[rensPols]

I think it's told clearly in the issue, but to summarise:

1. It is standard web practice, so it helps with familiarity
2. Reversing the children list gives problems with focus traversal
3. Reversing the children's list gives problems with their relative z-index (and therefore also their shadow)

[matthew-carroll]

@rensPols that's a description of a few downstream problems - but who needs this? Where are the companies or developers who are currently being held back by this limitation? And what are those people currently doing to get around this?

[rensPols]

@matthew-carroll Currently it's holding back the exDock-backend-client where we have multiple instances in which we use layered shadows to distinguish what is the parent of which.

I've shown the workaround we had to use for one of those issues in this comment on the original issue