Optimize performance of Flex.Item

[symbiogenesis]

Flex.Item was an IEnumerable but internally stored a collection of Children as a List.

It seemed much simpler to just inherit from a list and remove all of the IEnumerable implementation and related extension methods.

Added a benchmark to confirm the suspicion.

Before:

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
LayoutLotsOfItemsWithWrap	5.038 ms	0.1739 ms	0.5074 ms	171.8750	125.0000	62.5000	1.56 MB
LayoutLotsOfItemsNoWrap	4.280 ms	0.0836 ms	0.1057 ms	171.8750	125.0000	70.3125	1.56 MB

After:

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
LayoutLotsOfItemsWithWrap	4.384 ms	0.0805 ms	0.1930 ms	171.8750	156.2500	85.9375	1.57 MB
LayoutLotsOfItemsNoWrap	4.331 ms	0.0845 ms	0.0829 ms	171.8750	156.2500	85.9375	1.57 MB

[ghost]

Hey there @symbiogenesis! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[rmarinho]

/azp MAUI-UITests-public

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[rmarinho]

/azp run MAUI-UITests-public

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[pictos]

Inherit from List and uses the new on base methods is dangerous, if at some part of the code Item is cast to List<Item>, just the base.Add will be executed (The same for InsertAt). I believe having this class inherit from IEnumerable is to avoid this pitfall.

[symbiogenesis]

That is a fair point. Although, Flex.Item is an internal class and I don't see anywhere in the codebase that it is being cast to a list.

Probably some of the same benefits could be attained by just figuring out how to remove the FlexExtensions file and simply using the built-in list functions of the Children property directly.

Of course, the approach of this PR also simplifies the code a bit.