[Angular] Refine loaders logic, add env, config support + default loaders

[art-alexeyenko]

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

Description / Motivation

• Adjust sitecore.config flow for Angular
• Add process.env handling logic for Angular (work around process.env not being available in angular apps)
• Logically separate angular native logic (/app) and content-sdk integrations (/content-sdk) in the sample. This demonstrates the areas that fully support angular conventions and the areas that may not
• Add default loaders

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)