Utility belt

The recent update made it so that RCSS's output is directly inlinable as a react `style` (there's still the different that react style lets you do `width: 10`, while RCSS only allows `width: "10px"`, but we'll see). This will future-proof RCSS in case we ever switch to style object inlining, when/if it starts to make sense.

By then RCSS can stay as a collection of useful utilities for writing CSS in JS:
- `RCSS.rgba()`
- `RCSS.darken(someValue)`
- `RCSS.areaInSpritedImages(...)`
- `RCSS.generateKeyFramesWithInterpolation(...)` <- unique keyframe names would be neat

etc.

The issue describes a possible future collection of RCSS utilities, including rgba, darken, sprite-area, and interpolated keyframe helpers, but names no files, tests, or concrete acceptance criteria. First inspect the repository structure and existing utility implementations, then clarify which utility is in scope and how its behavior should be tested before starting.

Written by the indexing model from the issue text.