QA: add reusable merge conflict check workflow

This commit introduces a new, reusable workflow to check if PRs are in a "conflict state", and if so, add a "merge conflict" label and leave a comment on the PR.

The workflow uses a pre-existing action runner which offers a number of options to configure the action.

While the workflow itself is pretty straight-forward, it has been set up as a reusable workflow to centrally manage the default option settings for all repos.

Individual calling workflows can still overrule these defaults for the most pertinent options.

For the less pertinent settings, this overload has not been enabled, but these options can still be opened up to calling workflows at a later point in time if there is a need for it.

It also adds a workflow to use the reusable action for this repo.
Even though it is pretty unlikely there will be a multiple PRs open at the same time for this repo, running the action on each merge will provide some basic QA.

Refs:
* https://github.com/eps1lon/actions-label-merge-conflict

.github/workflows/merge-conflict-check.yml

@@ -0,0 +1,20 @@
+name: Check PRs for merge conflicts
+
+on:
+  # Check for new conflicts due to merges.
+  push:
+    branches:
+      - main
+  # Check conflicts in new PRs and for resolved conflicts due to an open PR being updated.
+  pull_request_target:
+    types:
+      - opened
+      - synchronize
+      - reopened
+
+jobs:
+  check-prs:
+    if: github.repository_owner == 'Yoast'
+
+    name: Check PRs for merge conflicts
+    uses: ./.github/workflows/reusable-merge-conflict-check.yml

.github/workflows/reusable-merge-conflict-check.yml

@@ -0,0 +1,62 @@
+name: Check PRs for merge conflicts
+
+on:
+  workflow_call:
+    # Note: the following inputs from the eps1lon/actions-label-merge-conflict
+    # action runner are not made available as inputs for end-user workflows at this time.
+    # - repoToken: set to secrets.GITHUB_TOKEN, which should be fine in all cases (except forks).
+    # - retryAfter: defaults to 120 seconds.
+    # - retryMax: defaults to 5 times
+    # - continueOnMissingPermissions: defaults to false
+    #
+    # If at some point in the future, there would be a need, these can still be added.
+    inputs:
+      dirtyLabel:
+        description: "Name of the label which indicates that the branch is dirty."
+        type: string
+        required: false
+        default: "merge conflict"
+      removeOnDirtyLabel:
+        description: "Name of the label which should be removed."
+        type: string
+        required: false
+        default: ''
+      commentOnDirty:
+        description: "Comment to add when the pull request is conflicting. Supports markdown."
+        type: string
+        required: false
+        default: "A merge conflict has been detected for the proposed code changes in this PR. Please resolve the conflict by either rebasing the PR or merging in changes from the base branch."
+      commentOnClean:
+        description: "Comment to add when the pull request is not conflicting anymore. Supports markdown."
+        type: string
+        required: false
+        default: ''
+
+jobs:
+  check-prs:
+    name: Merge conflict check
+
+    runs-on: ubuntu-latest
+    steps:
+      - name: "Create label if it doesn't exist"
+        uses: actions/github-script@v7
+        with:
+          script: |
+            try {
+              await github.rest.issues.createLabel({
+                ...context.repo,
+                name: ${{ inputs.dirtyLabel }}
+              });
+            } catch(e) {
+              // Ignore if labels exist already.
+            }
+
+      - name: Check PRs for merge conflicts
+        uses: eps1lon/actions-label-merge-conflict@v3
+        with:
+          repoToken: ${{ secrets.GITHUB_TOKEN }}
+          dirtyLabel: ${{ inputs.dirtyLabel }}
+          removeOnDirtyLabel: ${{ inputs.removeOnDirtyLabel }}
+          continueOnMissingPermissions: ${{ inputs.continueOnMissingPermissions }}
+          commentOnDirty: ${{ inputs.commentOnDirty }}
+          commentOnClean: ${{ inputs.commentOnClean }}