Fixes #41

Adds two new crates:

• processing_input, which handles extracting input data from Bevy when requested.
• processing_glfw, which is now just a common crate for GLFW related stuff, rather than having it separately versioned in examples and pyo3.

The goal here is to use Bevy's input machinery, rather than directly going from GLFW to processing. As such, when we poll events from GLFW, we feed them into Bevy as regular Bevy input events. We "flush" input by running the PreUpdate schedule which allows Bevy's internal input bookeeping to populate the respective ButtonInput, etc., resources.

One thing to note is that processing is kind of weird in that many input states are not reset every frame. So if you check for a pressed key it will remain "pressed" until a new key is pressed. For example, see the docs here: https://p5js.org/reference/p5/keyPressed/. As such, we need to track the most recent key press separately.

Something else worth noting is the strategy for allowing users to reference stuff like mouse_x in their Python sketches, where mouse_x is magically updated every frame. To do so, we set those as global variables in the scope of the draw function. This strategy likely has some gaps, and may need to be expanded on later.