Add Arthera Testnet to chains

[julienbrg]

Description

I added an artheraTestnet.ts file in the chains folder.

Additional Information

• I read the contributing docs (if this is your first contribution)

[changeset-bot]

⚠️ No Changeset found

Latest commit: e2136ad

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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[jxom]

Please add a changeset: pnpm changeset, and add to chains/src/index.ts

[tmm]

Chains should now be edited/added on the Viem repository.