Add viem test client example

[chrisx9z]

Summary

• add a README example for using viem createTestClient with a prool Anvil instance
• point the viem transport at the prool instance endpoint /1
• mention that prool lazily starts the instance on first request, with /1/start available for eager startup

Fixes #28.

Verification

• git diff --check
• rg -n "Viem Test Client|createTestClient|http\('http://localhost:8545/1'\\)|client.mine" README.md

Note: biome check does not process README.md with the current repository configuration, so this README-only change was verified with diff checks and content search.