Tokens: Severity icon color tokens

[andrew-ronaldson]

### Tasks
- [x] Unidentified
- [x] None
- [x] Minor
- [x] Moderate
- [x] Important
- [x] Critical

[lboehling]

added tokens to support for severity icon colors.

I set up the naming of these tokens to be consistent with the naming of our other status/non status icon colors. I didn't add hover/clicked tokens since I don't think they're needed now, but the naming is set up so that they can be added easily if we do need them.

Semantic color tokens added

Base dark color tokens added

Base light color tokens added

[andrew-ronaldson]

Thanks for getting these into the library!