[Tokens] Split the icons from `core.json`

[jpzwarte]

Provide a general summary of the issue here

Currently the icons are part of core.json, which also includes a whole bunch of "old" tokens. This will cause trouble when we move to split the old & new tokens. We should move the icons into another file.

👤 Your name

Jeroen

🧢 Your Product/Team

SLDS