Fix Ctrl+Enter bug for VS Code bindings

The Ctrl+Enter shortcut (Cmd+Enter on macOS) re-evaluates fidlbolt,
allowing the user to do so immediately rather than waiting for the 500ms
idle delay. The VS Code bindings have a mapping for Ctrl/Cmd+Enter
called addLineAfter, which was also getting triggered. This CL prevents
that by calling stopPropagation() in the event handler.

Change-Id: Ieb1131e783836a3790e99e2bbefe5bccbc5fc339
diff --git a/frontend/src/evaluator.ts b/frontend/src/evaluator.ts
index 9d28ec7..c48902c 100644
--- a/frontend/src/evaluator.ts
+++ b/frontend/src/evaluator.ts
@@ -121,6 +121,7 @@
       if ((event.ctrlKey || event.metaKey) && event.key === 'Enter') {
         clearTimeout(idleTimeout);
         this.run();
+        event.stopPropagation();
       }
     };