chore: add dependabot

[jeswr]

This is the standard dependant configuration that I use (e.g. in https://github.com/eyereasoner/eye-js/blob/main/.github/dependabot.yml).

This will create PRs weekly to update npm dependencies and GitHub packages. These PRs will not be merged automatically - to enable auto merging we would need to:

• Add an auto merge action like this one, and
• enable the auto merge setting in https://github.com/linkeddata/rdflib.js/settings, and
• enable branch protections in https://github.com/linkeddata/rdflib.js/branches that make the CI / tests "required" workflows. My default branch protections are:
  • Require a pull request before merging (all sub options are disabled)
  • Require status checks to pass before merging (once this is selected, you will be able to choose the required workflows)
  • Require branches to be up to date before merging
  • Require linear history
  • Do not allow bypassing the above settings