Fusion for general Lua(u) environments

[dphfox]

I've noticed over time that there's an increasing demand for using Fusion outside of Roblox. Currently this isn't possible without manually excluding parts of Fusion that deal with Roblox.

In the future, it would be nice to investigate separating out Fusion into a general-purpose 'core library' and a Roblox-specific 'addon library'. These would be packaged together for Wally and rbxm distributions to maintain backwards compatibility, but would enable new ventures for Fusion in other environments such as Love2D.

[dphfox]

Related to #40

[dphfox]

Darklua looks like a good candidate for solving the problem of require formatting. It may also be an elegant way to do feature flags that we currently model using constants at runtime.

[dphfox]

We could tie this into #40 and #254 as a bulk change, since this almost certainly will end up restructuring the project. Perhaps I'll merge these things together.

[dphfox]

Merged into #40