Remove Entity Component System entirely; add Mach object system instead

[emidoots]

This PR remove the Entity Component System from Mach entirely.

After multiple years of working on our ECS, and investigating and learning how other very popular ECS implementations work, I have ultimately not been happy with the tradeoffs that they make. ECS forces you, by necessity of its design goals, down a very particular path which has serious and massive implications for how developers must write code and the types of problems and software bugs they must deal with.

Instead, over the past few months we have been experimenting with a new Mach object system - which is not an ECS but which solves the same problems, and with very different tradeoffs and design decisions. Ultimately I believe this will be a better path forward for us.

Documentation

There is a new Object system section on the website which describes in-depth the key concepts and how it all works.

Note about this change

Note: the main branch of Mach is currently experimental, and this change:

• Will immediately break some mach.Core functionality - such as setting window titles - and will also break examples that aren't just rendering a triangle.
• After the change is merged, we'll work on migrating mach.Core's API to the new object system.
• After that, we'll work on updating gfx.Text, gfx.Sprite, sysaudio, sysgpu, and other APIs to use the new object system - with the goal first of getting all examples building again.

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