Add CI workflow to validate configuration files against JSON schema

Some of the assets used by the template workflows have JSON schema that defines the data structure in a machine
readable way. Validating the assets against the schema whenever they are modified ensures that no problems are introduced.
It is especially important to ensure the assets hosted in this repository are correct, since they will be distributed
from here to many other projects.

Author: per1234

.github/workflows/check-configs.yml

@@ -0,0 +1,36 @@
+name: Check Configuration Files
+
+# See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/check-configs.yml"
+      - "Taskfile.ya?ml"
+      - "**/dependabot.yml"
+  pull_request:
+    paths:
+      - ".github/workflows/check-configs.yml"
+      - "Taskfile.ya?ml"
+      - "**/dependabot.yml"
+  schedule:
+    # Run every Tuesday at 8 AM UTC to catch breakage resulting from changes to the JSON schema.
+    - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Install Task
+        uses: arduino/setup-task@v1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          version: 3.x
+
+      - name: Validate configuration files
+        run: task config:validate

Taskfile.yml

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+version: "3"
+
+tasks:
+  check:
+    desc: Check for problems with the project
+    deps:
+      - task: config:validate
+
+  config:validate:
+    desc: Validate configuration files against their JSON schema
+    vars:
+      # Last version with support for draft-04, used by Dependabot schema
+      AJV_CLI_VERSION: 3.3.0
+      # Source: https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/dependabot-2.0.json
+      DEPENDABOT_SCHEMA_URL: https://json.schemastore.org/dependabot-2.0
+      DEPENDABOT_SCHEMA_PATH:
+        sh: mktemp -t dependabot-schema-XXXXXXXXXX.json
+      DEPENDABOT_DATA_PATH: "**/dependabot.yml"
+    cmds:
+      - wget --quiet --output-document="{{.DEPENDABOT_SCHEMA_PATH}}" {{.DEPENDABOT_SCHEMA_URL}}
+      - npx ajv-cli@{{.AJV_CLI_VERSION}} validate -s "{{.DEPENDABOT_SCHEMA_PATH}}" -d "{{.DEPENDABOT_DATA_PATH}}"