[html] allow to configure html formatter for other languages

<!-- ⚠️⚠️ 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. --> This would allow users to, for example, force the default HTML formatter to format variations of the language, such as Django HTML. For example, VSCode won't run the default HTML formatter if the file I'm working on is associated with Django HTML. However, if I tell it it's just regular HTML the formatter will run fine, without any problem. This feature would allow to skip the middle step of telling the editor it's something else just to format the file. I think a good way to have this work would be to have something similar to the editor.defaultFormatter property, or just have it consider whatever is given to it the formatter that should be used.