This PR allows events to be move and copy assigned.

That's it. Definitely nothing else in 800 line changes.

Just kidding :

• Fixes style issues in dispatcher.h and dispatcher.cpp
• Adds unit tests checking that the types can be copy constructed and assigned as well as move constructed and assigned
• Changes the thread_local stop_event for event cancellation into a mutable bool inside of event_dispatch_t (yes, it still works)
• Adds various small optimizations in dispatcher.cpp
• Fixes some weird things (e.g. message_create_t::send taking with a non-const lvalue reference to dpp::message
• Makes VS2022 use PCH because why not

API remains the same for everything

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.