Updated EcsRx + Using Official MG libs

Historically this was built on top of an MG branch which targeted .net core, but now we are using .net 6 and the official MG nuget packages.

This also uses the latest EcsRx.

Updated to use latest EcsRx

This now uses the new version which does away with IEntityCollectionManager and has IEntityDatabase and IObservableGroupManager which do the same sort of things it used to do.

You are also now able to use the EcsRx persistence plugin if you want, which allows you to do complex data tasks in a simple and fluent way.

Not much changed really

Updated to use latest EcsRx, simplified some systems in example to use new IBasicSystem also updated dependencies to latest MonoGame (netcore version).

First Release!

This is by no means a finished project, it is a first stab at getting EcsRx integrated with monogame and it works with some basic conventions, so this first release is very much usable, but we hope the community will recommend new conventions and changes which will help build this out into a more feature rich integration.

It is recommended that you look at the VERY BARE BONES example provided with the repo as to how to use it, and also look at the core EcsRx docs, the discord channel is always available for more important questions.