[nextjs] Fix build errors when app uses personalize initializer, add utils, site submodule exports

[art-alexeyenko]

• This PR follows the Contribution Guide
• Changelog updated

Description / Motivation

NextJS version 13.3 has fixed some issues related to server imports in client context. This has caused some errors to occur when an import in a client component would also cause nextjs/server to be imported, like this one: vercel/next.js#48251
This PR moves SiteInfo, SiteResolver and tryParseEnvValue exports from '@sitecore-jss/sitecore-jss-nextjs/middleware' to separate submodules, in order to avoid these imports dragging server logic into a client bundle during build.

It's also possible to fix this issue by adding a next-config plugin to patch webpack configuration in a sample app. The patch would prevent loading 'fs' in client context (this would be a non-breaking approach), but that would be more of a workaround than a future-proof fix.

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate) - app builds without error with the changes applied

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)

[ambrauer]

Looks good to me, nice work 👍