Enable yamllint

Motivation:

More CI is better

Modifications:

Enable yamllint with overridden config.

Result:

Any yaml added will be checked

Author: PeterAdams-A

.github/workflows/pull_request.yml

@@ -10,7 +10,6 @@ jobs:
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
     with:
       license_header_check_project_name: "SwiftAsyncDNSResolver"
-      yamllint_check_enabled: false
       api_breakage_check_enabled: false
 
   unit-tests:

.yamllint.yml

@@ -0,0 +1,11 @@
+ignore: |
+  Sources/CAsyncDNSResolver/c-ares/
+  .cirrus.yml
+
+extends: default
+
+rules:
+  line-length: false
+  document-start: false
+  truthy:
+    check-keys: false  # Otherwise we get a false positive on GitHub action's `on` key