Fix the json assertion and update the CONTRIBUTING.md guidelines

[mjadach-iv]

Fixes #182

Summary

The code currently uses with { type: 'json' } for importing JSON modules, which is not valid ECMAScript syntax. The correct approach is to use assert { type: 'json' } for import assertions. This change is necessary to ensure compatibility with the official module specification and modern Node.js environments (follows 20.11+ requested in CONTRIBUTING.md).

With that change, it will work better with projects when being imported by wagmi.

Also, I updated the CONTRIBUTING.md as it talked about a not exiosting yarn build command.

How did you test your changes?

I built it using yarn build:packages and imported into an existing project using wagmi and the error from the issue dissapeared.

[cb-heimdall]

🟡 Heimdall Review Status

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[mjadach-iv]

Just pushed a commit with a verified signature.

[chenguzhen87]

solved?

[mjadach-iv]

solved?

Not yet.

Hey @spencerstock, can you please take a look at this PR?
I am tagging you here because I can see that you are the only contributor and I can't assign a reviewer to the PR