* Migrate NUnit 3.14.0 to 4.2.2
* Add NUnit.Analyzers 4.3.0
* Convert Classic Assert to Constraint Model
* Introduce Assert.Multiple to group assertions, ensuring all are evaluated even if some fail.
* Simplify test case source return types to IEnumerable without a type argument
* Remove unused using directives and added necessary ones.

Test logic is left unchanged except for 7 tests in Exception context.
Here all try...catch blocks are replaced with `Throws.` assertions.

- Implemented `CopyFrom` method in multiple classes for deep copying.
- Removed questions in code and implemented solution
- Updated `ICopyable` interface documentation.
- Introduced pattern matching for better readability
- Added new test methods in `CopyComponentTests` class for deep copying various calendar components.
- Fixed broken SerializationTests.AttendeesSerialized() (Corrected attendee names and fixed typos in assertions.)
- Refactored `CalendarObjectBase` (should eventually become abstract)
- Added `ExcludeFromCodeCoverage` attribute to `CalendarObjectList`.
- Updated `Ical.Net.csproj` to latest C# version.

Fixes ical-org#149

* Removed redundant check in `Attachment.cs` `CopyFrom` method.
* Updated `Period.cs` and `Trigger.cs` to use `Copy<IDateTime>()` method.
* Modified `Alarm` class to implement `IComparable<Alarm>` with `CompareTo` method.