refactor: Nullable and Modernization for Stride.Core. & Stride.Core.AssemblyProcessor

[Kryptos-FR]

PR Details

In the same spirit of #2266, modernize Stride.Core and Stride.Core.AssemblyProcessor.

Description

1. Enable nullable on both projects.
2. Use new language features such as collection initialization, pattern matching, range and indexers, etc.
3. Enable implicit usings, and use file-scoped namespaces.

Related Issue

#2155
#2156

Types of changes

• Docs change / refactoring / dependency upgrade
• 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)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.

[Eideren]

Thanks !