feat(flame_3d): initial implementation of 3D support

[wolfenrain]

Description

This PR brings a new package to the Flame ecosystem: flame_3d.

I have added a clear description of the goals of the package and it's roadmap as part of the PR but for transparency sake I'll add the excerpt in this PR description as well:

This package provides 3D support for Flame but it depends on the still experimental 
[Flutter GPU](https://github.com/flutter/flutter/wiki/Flutter-GPU) which in turn depends on 
Impeller. The Flutter GPU is currently not shipped with Flutter so this package wont work without 
following the prerequisites steps.

Because we depend on Flutter GPU this package is also highly experimental. Our long term goal is to
eventually deprecate this package and integrate it into the core `flame` package, for more 
information on this see the [Roadmap](https://github.com/flame-engine/flame/blob/main/packages/flame_3d/ROADMAP.md).

This package does not guarantee that it will follow correct [semver](https://semver.org/) versioning
rules nor does it assure that it's APIs wont break. Be ready to constantly have to refactor your 
code if you are planning on using this package in a semi-production environment, which we do not
recommend. 

Documentation and tests might be lacking for quite a while because of the potential constant changes
of the API. Where possible we will try to provide in-code documentation and code examples to help
developers but our main goal for now is to enable the usage of 3D rendering within a Flame 
ecosystem.