Add Dependabot to Update Dependencies

[nelsonic]

Create a file with the following path: .github/dependabot.yml

With the following contents:

version: 2
updates:
- package-ecosystem: mix
  directory: "/"
  schedule:
    interval: daily
    time: "07:00"
    timezone: Europe/London

Docs: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

[nelsonic]

PR: #159