This makes bootstrapping the project much, much simpler, and should
enable easier builds in a hypothetical CI. However, it had many downstream
effects on CMake, linking, and documentation, mostly positive. Notably:

* No more manual `find_package()` implementations- unequivocal positive.
* Building directions should now be much clearer- unequivocal positive.
* Since we're no longer arbitrarily consuming imgui code, we can't
  reference imgui_demo.cpp directly. I copied the log window
  implementation from the imgui demo directly into the overlay,
  which is actually closer to imgui's recommendations anyway.
* `vcpkg` by default installs DLLs, and we were previously statically
  linking spdlog. `sf4e` now injects the spdlog and fmt DLLs when preparing
  SF4 for execution.
* One of our dependencies isn't actually available via `vcpkg`, but it's
  no more difficult than CMake to declare new dependencies.
* Some of the CMake rules have been optimized (ex. installing no longer
...