Add a setting to automatically detect the language of a new file no more than once.

<!-- ⚠️⚠️ 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. --> I notice when I create a blank file and start typing, it detects the language and is usually correct so I don't change it. But when I keep typing, it changes the language again. There should be a setting not to turn off auto detect, but to do so no more than once.