These blocks implement pen features as found in Scratch 2.0
Supporting changes include:
- `Target` is now an event emitter
- `RenderedTarget` now emits an event when it moves
- `Target` can now store arbitrary "extra" data, called "custom state"
in the code, using a `Target`'s `setCustomState` and `getCustomState`
methods. This is used to store per-target pen state without requiring
`Target` or `RenderedTarget` to know anything about the pen.
- `Cast` can now cast to an RGB color object.
- `Color` now has functions to convert between RGB and HSV, constants
for for black & white, and a `mixRgb` function to lerp between two
colors.
