[draft] CMake support

[VirtualBrightPlayz]

All the c/cpp files compile, but the shaders are an issue I spotted.
To "compile" shaders, you will need https://github.com/microsoft/DirectXShaderCompiler and https://github.com/KhronosGroup/SPIRV-Cross.
Make sure to set DXC_CMD to the dxc.exe file and SPVC_CMD to spirv-cross.exe file (assuming Windows).

Note:
Due to the way pge is designed, the compiled shaders don't work.

This also updates assimp

[VirtualBrightPlayz]

Ok, the shader compiler I used (dxc and spirv-cross) are no longer required (or used), and the engine and game compile and run (Windows only for now), but you need to update the engine submodule to this commit: juanjp600/pge@655f8a0 for it to function without crashing.

This also makes assimp a submodule, and adds three new compiler directives for the engine, PGE_D3D for Direct3D support, PGE_VKR for Vulkan, and PGE_OGL for OpenGL.

I also removed the OpenVR submodule, as I didn't mean to add it.

[VirtualBrightPlayz]

I've been looking into supporting Linux with this (was going to make a new PR), however there is a bug in both gcc and clang that prevents PGE's String class from compiling.
I'm still looking for obvious workarounds, but I think the "workaround" is custom patches to gcc (clang doesn't support some of the c++20 features yet) or rewrite parts of the String code to work on linux. Not gonna make a new issue on PGE regarding Strings, as juanjp600/pge#56 exists, which might fix this depending on implementation.
clang/llvm issue gcc issue

macOS support with CMake is unknown as I don't own a mac.

TODO: futureproof the CMake files with removal of TARGET_RUNTIME_DLLS