Provide way to control render order of plugins registered via registerPlugin

[ryanwelcher]

Currently, the order in which plugins are rendered inside of the PluginArea component is based on the order in which they are registered. As more SlotFills are created and exposed, the ability to set a priority for render order will become more important. SlotFills like PluginDocumentSettingPanel are going to highlight this need once 5.3 is released.

My proposal is to add a priority property to the settings object passed to registerPlugin that will have a default value of 10. We can then sort the array of registered plugins based on that priority. This is a paradigm that will be familiar to most WordPress developers.

It should be noted that this will not affect any of the SlotFills directly.

[ryanwelcher]

#16384 is a first attempt to implement this.