[React] [Next.js] Strongly typed SitecoreContext value

[illiakovalenko]

Description / Motivation

• <SitecoreContext/> from sitecore-jss-react package should have exact type for context state
• React sample should work as expected in connected/headless/integrated modes
• Nextjs sample should work as expected using SSG/SSR modes
• <Layout/> component shouldn't run render when it's unnecessary
• (Additional issue which I noticed and fixed) Previously when you tried to switch language on the /styleguide page react re-rendered <Layout /> component twice. Now it's fixed and it was happening b\c we rendered array of routes, see https://github.com/Sitecore/jss/pull/841/files#diff-b6f3b4055f9147512d69014aa8a1d1df8d4c129713f504a4ca597c75e224bb49R37-R39
  updateSitecoreContext should trigger update for layout. Test it using:
  // withSitecoreContext({ updatable: true });
  // this.props.updateSitecoreContext(freshLayoutData);

BREAKING CHANGE

• SitecoreContext accepts layoutData instead of context
• SitecoreContext has strict state type instead of any

Testing Details

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update (non-breaking change; modified files are limited to the /docs directory)

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[CobyPear]

Looks good!