Add new transparency tokens

[kyledurand]

This adds the ability to generate gray tokens with alpha values for the new transparency subset

[aaronccasanova]

This is a sufficient starting point for applying alpha values to the gray palette and unblocking beta overrides. I'm not sure the pattern is very scalable as we'd likely have to update each shade to a function that returns an rgba to establish a convention and consistency.

Two approaches I would like to see explored is a simple toRgba util that updates the alpha (e.g. toRgba(gray[2], 0.5)) or breaking up palette colors into two parts (e.g. a grayParts object with rgb values and a gray object using the parts and applying alpha).

Since accessing these tokens in JS or the build assets is always a key/value pair, I think we are fine to proceed with this strategy and update to the above or something else in the future.

[yurm04]

looks good as a first/quick solution. Agree with Aaron that this could be further optimized but let's ship this now and come back to it 💯 🚢

[kyledurand]

Yeah, now that we have a transparency theme we need the system to be more flexible at scale, agreed. This means a system change at some point but for now this works within the current system