Consider supporting shared token sets across themes

**Is your feature request related to a problem? Please describe.**

The Style Dictionary setup doesn't currently reflect the way themes are defined in Tokens Studio.

A theme in Tokens Studio is an arbitrary collection of token sets. This allows us to have shared token sets across themes. For example, Light and Light HC themes can both depend on a single `theme-semantics-light` token set for defining the asymmetrical semantic tokens, instead of having both `theme-semantics-light` and `theme-semantics-light-hc` with duplicate contents.

The latter is not desirable as it requires us to manually keep both token sets in sync instead of having a single source of truth.

**Describe the solution you'd like**

- Light and Light HC theme would depend on a shared token set for defining semantic tokens, `theme-semantics-light`.
- Dark and Dark HC theme would depend on a shared token set for defining semantic tokens, `theme-semantics-dark`.

**Describe alternatives you've considered**

So far, we've reverted Tokens Studio to use four semantic token sets:

- `theme-semantics-light`
- `theme-semantics-dark`
- `theme-semantics-light-hc`
- `theme-semantics-dark-hc`