fix: DH-20953: AG Grid styling only worked when deephaven.ui installed (#1271)

- Adding a generic widget-container class that can be used by any widget
- See deephaven/web-client-ui#2581 for the
web-client-ui updates
- Not removing the old class so it doesn't break in the interim
- Set up deephaven.ui to handle `.widget-container` correctly

Author: mofojed

plugins/ag-grid/src/js/src/components/AgGridWidget.tsx

@@ -46,7 +46,7 @@ export function AgGridWidget(
   const table = useWidgetFetch(dh, fetch);
 
   return table != null ? (
-    <div className="ui-table-container">
+    <div className="ui-table-container widget-container">
       <AgGridView table={table} settings={settings} agGridProps={agGridProps} />
     </div>
   ) : (

plugins/ui/src/js/src/styles.scss

@@ -67,6 +67,7 @@
         > .ui-table-container:only-child
         > .iris-grid:only-child
     ),
+  &:has(.dh-inner-react-panel > .widget-container:only-child),
   &:has(.dh-inner-react-panel > .chart-wrapper:only-child) {
     // remove the default panel padding when grid or chart is the only child
     padding: 0 !important; // important required to override inline spectrum style