Add reusable workflow to check no important files changed (#157)

.github/workflows/check-no-important-files-changed.yml

@@ -0,0 +1,65 @@
+name: No important files changed
+
+on:
+  workflow_call:
+    inputs:
+      repository:
+        description: "The pull request owner's repository"
+        required: true
+        type: string
+      ref:
+        description: "The pull requests's reference"
+        required: true
+        type: string
+
+permissions:
+  pull-requests: write
+
+jobs:
+  no_important_files_changed:
+    name: No important files changed
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
+        with:
+          repository: ${{ inputs.repository }}
+          ref: ${{ inputs.ref }}
+
+      - name: Check if important files changed
+        id: check
+        run: |
+          set -exo pipefail
+
+          git remote set-branches origin main
+          git fetch --depth 1 origin main
+
+          git diff --diff-filter=M --name-only origin/main
+
+          for changed_file in $(git diff --diff-filter=M --name-only origin/main); do
+            slug="$(echo "${changed_file}" | sed --regexp-extended 's#exercises/[^/]+/([^/]+)/.*#\1#' )"
+            path_before_slug="$(echo "${changed_file}" | sed --regexp-extended "s#(.*)/${slug}/.*#\\1#" )"
+            path_after_slug="$( echo "${changed_file}" | sed --regexp-extended "s#.*/${slug}/(.*)#\\1#" )"
+            config_json_file="${path_before_slug}/${slug}/.meta/config.json"
+
+            if ! [ -f "${config_json_file}" ]; then
+              # cannot determine if important files changed without .meta/config.json
+              continue
+            fi
+
+            changed=$(jq --arg path "${path_after_slug}" '[.files.test, .files.invalidator, .files.editor] | flatten | index($path) != null' "${config_json_file}")
+            echo "important_files_changed=${changed}" >> "$GITHUB_OUTPUT"
+          done
+
+      - name: Suggest to add [no important files changed]
+        if: steps.check.outputs.important_files_changed == 'true'
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea
+        with:
+          script: |
+            const body = "This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.\n\nIf this PR does **not** affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), **please add the following to the merge-commit message** which will stops student's tests from re-running. Please copy-paste to avoid typos.\n```\n[no important files changed]\n```\n\n For more information, refer to the [documentation](https://exercism.org/docs/building/tracks#h-avoiding-triggering-unnecessary-test-runs). If you are unsure whether to add the message or not, please ping `@exercism/maintainers-admin` in a comment. Thank you!"
+            github.rest.issues.createComment({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: body
+            })