Create serialize feature for bevy_ui

[pablo-lua]

Objective

• Fixes Create a feature to Serialize in bevy_ui #11119

Solution

• Creation of the serialize feature to ui

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Changelog

Changed

• Changed all the structs that implement Serialize and Deserialize to only implement when feature is on

Migration Guide

• If you want to use serialize and deserialize with types from bevy_ui, you need to use the feature serialize in your TOML

[dependencies.bevy]
features = ["serialize"]

[pablo-lua]

There is probably something missing here and there with the reflect. Noticed that some structs have Default and do not reflect it. Well, didn't touch it for now