fix: add back the default tooltip textStyle settings. Without those s…

…ettings, the tooltip text will follow the website settings if user specified `tooltip.formatter`, which is not expected.

src/component/tooltip/TooltipModel.ts

@@ -169,6 +169,8 @@ class TooltipModel extends ComponentModel<TooltipOption> {
             // otherwise it will always override those styles on option.axisPointer.
         },
         textStyle: {
+            color: '#666',
+            fontSize: 14
         }
     };
 }