Update datasource.ts

Author: vytautas-karpavicius

src/datasource.ts

@@ -8,6 +8,7 @@ import { map } from 'lodash';
 export class DataSource extends DataSourceWithBackend<TrinoQuery, TrinoDataSourceOptions> {
   constructor(instanceSettings: DataSourceInstanceSettings<TrinoDataSourceOptions>) {
     super(instanceSettings);
+    this.variables = new TrinoDataVariableSupport();
     // give interpolateQueryStr access to this
     this.interpolateQueryStr = this.interpolateQueryStr.bind(this);
   }