Feat/custom assertions

[codefriar]

Adds a 'shadow' Assert class that facilitates developers adding custom assertion methods that can still be cleanly read as Assert.methodName

Closes #104

Summary by CodeRabbit

• New Feature: Introduced a new Assert class with custom assertion methods for various conditions, enhancing the testing capabilities.
• Refactor: Updated return types of several methods across multiple classes to improve type safety and clarity.
• Documentation: Revised documentation for numerous classes, improving descriptions of methods and their return types.
• Test: Added visibility to certain enum constants for testing purposes.
• Chore: Minor changes in formatting and phrasing in the codebase's documentation.