Feat: adds LazyLoadingSection component - perf (#2521)

## What's the purpose of this pull request?

Adds `LazyLoadingSection` component, it will be responsible for lazy
loading Sections that are out of the viewport.
 * It achieves this by:
 * 1. Using the IntersectionObserver API for Sections below the fold.
* 2. Checking the UI context for Sections that are not in the viewport,
such as the CartSidebar and RegionModal.

We will be implementing this in the `RenderSectionsBase` as part of our
upcoming tasks. This component is not being applied in isolation due to
concerns about increased Cumulative Layout Shift (CLS).

## How to test it?

1. You can use this [preview
link](https://starter-git-feat-lazy-loading-section-sfs-1508-faststore.vercel.app/),
where the `LazyLoadingSection` is temporary applied or locally add it in
`RenderSectionsBase` component.

``` 
 <SectionBoundary key={`cms-section-${name}-${index}`} name={name}>
      <LazyLoadingSection sectionName={name}>
           <Component {...data} />
      </LazyLoadingSection>
 </SectionBoundary>

```

2. Inspect the code, use the React extension for this test:
- search for `Footer`, you won't find it until you scroll the page until
the `Newsletter` section
<img width="1144" alt="image"
src="https://github.com/user-attachments/assets/46e2eca0-1ce3-473c-9153-29ddb731881a">

<img width="1164" alt="image"
src="https://github.com/user-attachments/assets/3132efca-38c5-4e41-b8e2-ea69b2adbf9e">



https://github.com/user-attachments/assets/bbfffbe0-39b8-4b6b-b86d-04d4a7ab1e6b


### Starters Deploy Preview
vtex-sites/starter.store#586

## References
#2404

---------

Co-authored-by: Pedro Soares <32311264+pedromtec@users.noreply.github.com>

packages/core/src/components/cms/RenderSections.tsx

@@ -1,13 +1,24 @@
-import { ComponentType, PropsWithChildren, memo, useMemo } from 'react'
+import {
+  ComponentType,
+  PropsWithChildren,
+  ReactNode,
+  memo,
+  useMemo,
+} from 'react'
 
-import SectionBoundary from './SectionBoundary'
 import { Section } from '@vtex/client-cms'
+import SectionBoundary from './SectionBoundary'
+import ViewportObserver from './ViewportObserver'
+
+import { useUI } from '@faststore/ui'
 
 interface Props {
   components: Record<string, ComponentType<any>>
   sections: Array<{ name: string; data: any }>
 }
 
+const SECTIONS_OUT_OF_VIEWPORT = ['CartSidebar', 'RegionModal']
+
 const useDividedSections = (sections: Section[]) => {
   return useMemo(() => {
     const indexChildren = sections.findIndex(({ name }) => name === 'Children')
@@ -21,6 +32,39 @@ const useDividedSections = (sections: Section[]) => {
   }, [sections])
 }
 
+/**
+ * This component is responsible for lazy loading Sections that are out of the viewport.
+ * It achieves this by:
+ * 1. Using the IntersectionObserver API for Sections below the fold.
+ * 2. Checking the UI context for Sections that are not in the viewport, such as the CartSidebar and RegionModal.
+ *
+ * @param sectionName
+ * @returns
+ */
+export const LazyLoadingSection = ({
+  sectionName,
+  children,
+}: {
+  sectionName: string
+  children: ReactNode
+}) => {
+  const { cart: displayCart, modal: displayModal } = useUI()
+
+  if (SECTIONS_OUT_OF_VIEWPORT.includes(sectionName)) {
+    const shouldLoad =
+      (sectionName === 'CartSidebar' && displayCart) ||
+      (sectionName === 'RegionModal' && displayModal)
+    if (!shouldLoad) {
+      return null
+    }
+
+    return children
+  }
+  return (
+    <ViewportObserver sectionName={sectionName}>{children}</ViewportObserver>
+  )
+}
+
 const RenderSectionsBase = ({ sections = [], components }: Props) => {
   return (
     <>
@@ -37,7 +81,7 @@ const RenderSectionsBase = ({ sections = [], components }: Props) => {
         }
 
         return (
-          <SectionBoundary key={`cms-section-${index}`} name={name}>
+          <SectionBoundary key={`cms-section-${name}-${index}`} name={name}>
             <Component {...data} />
           </SectionBoundary>
         )

packages/core/src/components/sections/Hero/Hero.tsx

@@ -60,6 +60,7 @@ const Hero = ({
             width={360}
             height={240}
             sizes="(max-width: 412px) 40vw, (max-width: 768px) 90vw, 50vw"
+            priority={true}
           />
         </HeroImage.Component>
         <HeroHeader.Component