World concurrency fixes, structural change annotations, and code cleanup

[LilithSilver]

• Fixes some concurrency issues with World to make it thread-safe (outside of structural change and parallel query methods)
  • Of note, adds necessary locking mechanisms to World.Events.cs and world.Query() (the query caching behaviour)
• Annotates structural changes with [StructuralChange] in World.cs
• Annotates safe world extension methods with [WorldExtension]
• Changes World API to officially support null class components through nullables, and fixes many (not all) nullable issues in the project
• Makes Group<T0, ...>.Hash and .Types public
• Minor improvements to documentation and code style based on the current .editorconfig, mostly focusing on World.cs
• Compiler and intellisense warning fixes

[genaray]

Nice, great job! :)