[create-sitecore-jss] Replace sass-alias with Next.js 16 compatible importer

[MenKNas]

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added

Description / Motivation

Issue

Next.js 16 upgrade broke Sass compilation in nextjs-sxa template:

• sass-alias package uses legacy Sass API incompatible with Next.js 16
• Node.js built-in modules being bundled for client causing build errors

Fix

• Replaced sass-alias with custom importer implementing Next.js 16 Sass API
• Added webpack config to exclude Node.js built-ins from client bundle
• Removed sass-alias dependency

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate)

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)