Add support for container tokens

[joshblack]

We have the following size variants that exist in the system: #8370 (comment)

For each one, we would like to make a container token to match it to make it easier to author styles both on our end and for consumers.

The tokens should be introduced in the carbon-components package and the naming scheme should follow: $size-<name>, for example $size-sm, $size-md, etc.