when clause contexts for emmet editing

<!-- ⚠️⚠️ 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. --> Emmet commands are often nicer than some of the similar functionality than VSCode's editing, so sometimes you want those keybindings to override the default behavior of VSCode, however those keybindings are useless in files that don't utilize Emmet. eg: Emmet's comment vs vscode's comment. It would be nice to have a when clause context that can identify when the cursor is in a language supported by emmet. `editorLangId` doesn't accomplish this because it cannot differentiate the cursor being in the HTML section or JS/PHP/etc. section of a Vue/PHP/etc. file. The context could have a boolean to specify if the current code context falls under the `emmet.includeLanguages` and could evaluate to the context. I could also see a context that specifies the underlying type of the current code being edited could be useful for other cases, though I'm not sure what that would mean. I'd guess it'd be the lowest language-in-a-language for syntax highlighting.