Restructure repository; create "extras" package

[hmans]

During the development of 0.4, I converted this repository to a Lerna-based monorepo and started creating individual micropackages for specific features (like text rendering, framework glue, and so on.) I have since realized that this model is not going to scale terribly well (as long as the project is primarily maintained by myself). For this reason, I would like to make the following changes:

• Create a separate "extras" package that will, for now, house all the various extra stuff implemented on top of three-elements' core package (loaders, text rendering, etc.), similar to how react-drei relates to react-three-fiber
• House this new package in a separate repository (that may at some point have a different set of maintainers from the core package)
• move most/all of the various micropackages from this repository to the new package

And ideally I'd like to do all of this before releasing 0.4.