Dependabot version updates

[cameel]

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. 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.