fix(back-top): reset native button styles and support custom children

Add border/padding reset for the button element. Add _custom modifier
to strip default styling when children are provided. Offset custom
trigger demo to prevent overlap with the basic example.

Author: wangdicoder

components/back-top/demo/custom.md

@@ -7,7 +7,7 @@ Use custom children to replace the default button.
 ```jsx live
 () => {
   return (
-    <BackTop visibilityHeight={100}>
+    <BackTop visibilityHeight={100} style={{ bottom: 100 }}>
       <div style={{
         width: 40,
         height: 40,

components/back-top/index.tsx

@@ -27,7 +27,9 @@ const BackTop = (props: BackTopProps): JSX.Element | null => {
   } = props;
   const configContext = useContext(ConfigContext);
   const prefixCls = getPrefixCls('back-top', configContext.prefixCls, customisedCls);
-  const cls = classNames(prefixCls, className);
+  const cls = classNames(prefixCls, className, {
+    [`${prefixCls}_custom`]: !!children,
+  });
   const [visible, setVisible] = useState(true);
 
   const getDistanceFromTop = useCallback((): number => {

components/back-top/style/_index.scss

@@ -12,5 +12,14 @@
   justify-content: center;
   align-items: center;
   cursor: pointer;
+  border: none;
+  padding: 0;
   transition: opacity 200ms linear 0s, visibility;
+
+  &_custom {
+    background: none;
+    width: auto;
+    height: auto;
+    border-radius: 0;
+  }
 }