[Emmet] Include should be more like DocumentSelector

Currently, the Emmet extension has a setting called `emmet.includeLanguages` which just targets language IDs to map to the Emmet-capable language ID. I think it would be very useful if instead you could use something more like `DocumentSelector`, which also allows [`DocumentFilter`](https://code.visualstudio.com/docs/extensionAPI/vscode-api#DocumentFilter) instead of just language ID.