renderer_vulkan: Move instance initialization to a separate file

Simplify Vulkan's backend initialization code by moving it to a separate
file, allowing us to initialize a Vulkan instance from different
backends.

src/video_core/CMakeLists.txt

@@ -258,6 +258,8 @@ add_library(video_core STATIC
     textures/texture.h
     video_core.cpp
     video_core.h
+    vulkan_common/vulkan_instance.cpp
+    vulkan_common/vulkan_instance.h
     vulkan_common/vulkan_library.cpp
     vulkan_common/vulkan_library.h
     vulkan_common/vulkan_wrapper.cpp