chore: add scoped viem dependabot updates for /typescript

[Bortlesboat]

Summary

• add a Dependabot config at the repo root for the /typescript workspace
• scope updates to viem only so chain additions land without broad dependency churn
• keep the PR volume low with one weekly update and major versions ignored

Testing

• python -c "import pathlib; import yaml; p=pathlib.Path(r''C:/Users/andre/.config/superpowers/worktrees/x402/codex-x402-viem-lockfile-freshness-1971/.github/dependabot.yml''); data=yaml.safe_load(p.read_text()); print(data[''version'']); print(data[''updates''][0][''allow''][0][''dependency-name''])"

Closes #1971

[vercel]

@Bortlesboat is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.