Recommend augmenting csstype instead of casting to `any`

[cpmsmith]

Currently, the docs recommend just casting to any whenever using CSS variables in TypeScript, but you don't have to do that. If you use module augmentation instead, you still get the benefits of typechecking, including on your CSS variables!

And here's a CodeSandbox to prove that it works: https://codesandbox.io/s/framer-motion-csstype-augmentation-shmjq

Note how it correctly flags the misspelled CSS variable:

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