Problem
Support the new external files option that TS plugins can provide: microsoft/TypeScript#15308

We currently allow TSServer plugin extensions to register themselves for custom language modes. The angular extension for example could register the angular plugin with support for the ng-html language. There were a few problems with the current implementation:

1. We start automatically syncing all ng-html files with TSServer.
2. If the angular plugin is not active, TS treats these ng-html files as TypeScript code

Potential Fix
On potential fix we discussed previously: add a plugin property on open requests. TypeScript would look at the plugin property and only start validating the file if has the corresponding plugin.