Dependabot version updates

We can have dependabot ping us when dependencies need to be updated. We have that enabled for security vulnerabilities but not for regular updates. See [Configuring Dependabot version updates](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuring-dependabot-version-updates). This will require adding a `dependabot.yml` to the repo.

Before doing this, check if this actually makes sense in our case though. Does dependabot pin specific versions? Since solc-js is a library, we want to support a wide range of versions and not impose specific versions on applications.

Start with the repository's dependency declarations and the linked GitHub documentation on configuring Dependabot version updates. Determine whether enabling regular updates would pin solc-js dependencies or otherwise restrict the supported version range. Done means documenting that decision and, if appropriate, adding the repository-root dependabot.yml configuration.

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