Add a `do not merge` label that prevents merging

^{Originally posted by **Snailedlt** October 4, 2022}
Would it be useful to have a `do not merge` label?
Maybe the PR could have a job that fails if the label is applied too.

Example implementation from here:
```yaml
name: Do Not Merge

on:
pull_request:
types: [synchronize, opened, reopened, labeled, unlabeled]

jobs:
do-not-merge:
if: ${{ contains(github.event.*.labels.*.name, 'do not merge') }}
name: Prevent Merging
runs-on: ubuntu-latest
steps:
- name: Check for label
run: |
echo "Pull request is labeled as 'do not merge'"
echo "This workflow fails so that the pull request cannot be merged"
exit 1
```