[mod] UI smooth scroll added / CSS scroll-behavior

This PR improves the UX by making auto-scroll more smoother.  The css is changed
so all the auto-scroll will be smoother but User-scroll will not be influenced.

  The scroll-behavior CSS property sets the behavior for a scrolling box when
  scrolling is triggered by the navigation or CSSOM scrolling APIs.[1]

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/scroll-behavior

searx/static/themes/simple/src/less/style.less

@@ -66,6 +66,8 @@ html {
   background-color: var(--color-base-background);
   padding: 0;
   margin: 0;
+
+  scroll-behavior: smooth;
 }
 
 body,