QueryField: Handle autocomplete better (#81484)

* extract out function + add unit tests

* add feature toggle and default it to on

packages/grafana-data/src/types/featureToggles.gen.ts

@@ -174,4 +174,5 @@ export interface FeatureToggles {
   onPremToCloudMigrations?: boolean;
   alertingSaveStatePeriodic?: boolean;
   promQLScope?: boolean;
+  slateAutocomplete?: boolean;
 }