Slate v2 History isHistory Recovery

Goal

Restore the small legacy History.isHistory(...) helper on the current slate-history package surface.

Completed

• added packages/slate-history/src/history.ts
• exported History from the package root
• proved History.isHistory(...) in history-contract.ts
• updated the history front-door docs to name the helper

Verification

• yarn workspace slate-history run test
• yarn test:mocha
• yarn lint:typescript