integrations/viem: doctests

[CedarMist]

Adds tests to ensure the README.md examples from integrations/viem-v2 can be imported.

This will catch any glaring errors. I think we should extend this to all of the packages in this repo.

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[lukaw3d]

Nice!

---

If we'll need snippets that skip some initialization code - copying a comment from slack:
For previous attempts at testing code in markdown we pulled code in from tests:

• generated documentation using vuepress snippets https://v1.vuepress.vuejs.org/guide/markdown.html#import-code-snippets
  (Ignore #region and #endregion comments for snippets when including whole file vuejs/vuepress#2663)

• docs.oasis.io generated using docusaurus + https://github.com/oasisprotocol/docs/blob/ded70ed/src/remark/code-block-snippets.ts

  https://docs.oasis.io/rofl/app#testing-it-on-sapphire-localnet
  -> https://github.com/oasisprotocol/oasis-sdk/blob/d23473d/docs/rofl/app.md?plain=1#L122
  -> https://github.com/oasisprotocol/oasis-sdk/blob/d23473d/examples/runtime-sdk/rofl-oracle/src/main.rs#L24-L35

New idea that would work with npm and github markdown without generating them:

• keep code as is in markdown
• add same code to files in tests
• add test that checks that code is found in a test (ignore indentation/whitespace)