Add Github Actions dependabot

[james-s-w-clark]

Just like https://github.com/awslabs/smithy/blob/main/.github/dependabot.yml

[james-s-w-clark]

PRs shouldn't be frequent, but you can also have:

.github/workflows/auto-merge.yml

name: auto-merge

on:
  pull_request:
    branches:
      - main

jobs:
  automerge:
    runs-on: ubuntu-latest
    permissions:
      pull-requests: write
      contents: write
    if: ${{ github.actor == 'dependabot[bot]' }}
    steps:
      - uses: fastify/github-action-merge-dependabot@v3.0.2
        with:
          github-token: ${{ secrets.GITHUB_TOKEN }}

FWIW, if anyone tries to make an account to abuse this:

Username may only contain alphanumeric characters or single hyphens, and cannot begin or end with a hyphen. Username dependabot[bot] is not available.

So the only concern is adding and extra file to remove some tiny toil 😆