Add GitHub Actions workflow to automatically merge Dependabot PRs

[luisaveiro]

The repo includes a Dependabot configuration YAML file to update GitHub Actions & npm package dependencies on a weekly interval. Currently, approval and merging of PRs are done manually.

Utilising the dependabot/fetch-metadata GitHub Actions we can determine the PR's semantic versioning, minor, patch, and major. Preferably we would merge in patch and minor semantic version PRs automatically. The dependabot/fetch-metadata GitHub Actions readme includes an example workflow for auto-approving and enabling auto-merge.