[html] Add value proposals for rel attribute

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> VSCode doesn't provide any support for autocompleting rel attribute in HTML.  If added, autocompletion for rel attribute in HTML will make the editing experience more consistent.